Interspecific Hybridization in Gucuinis

JOHN it 1)EAKIN. C. W. IIOIIN AX) THOMAS W. \VIIITAKE11

Introduction tis (9), and for resistance to several diseases by Leppik (18, 19, 20). Corley (7), Sowell Among the genera of the Cucurbitaceae et al. (32), and Leppik et al. (21) suggest (Gourd Family) used for human food, the that resistance to still other destructive pests genus Cticnnns contains two species of great may be found within the genus in species Cuctirnis mclo L., the economic importance. other than C. snelo. muskmelons, and C. sativus L., the cucum- An exchange of genes between the cul- bers, are widely grown for their edible fruits tivated and feral or semi-feral species of or seeds in nearly all of the tropical, semi- Cncnwis would open a vast potential re- tropical, and temperate regions of the world. source of variability for exploitation by A few other species such as C. angtiria (the breeders attempting to improve muskmelons gherkins) and C. rnctnliferns (the African and cucumbers. The exploratory studies Horned Cucumber) are grown on a smaller reported here failed to achieve that objec- scale for food or ornament (38). tive, but some successful species crosses Cncumis niebo is extremely variable. Some were achieved. These studies extend our forms produce round fruits ranging in size knowledge of this large but little-known from 1 inch in diameter in feral forms to genus and will aid the work of other inves- several inches in diameter and weighing up tigators in achieving crosses between C. to 20 pounds in cultivated varieties (can- mebo or C. sativns with the noncultivated taloup, Honeydew, Crenshaw, Persian, and species. Golden Beauty Casaba). Other forms are moderately to extremely elongated, reaching Literature lengths of 2 to 3 feet in some cultivars such as Banana melon and Chinese cucumber. A major deterrent to investigations of The species is equally variable in other species crosses in Csicurnis has been the vegetative and fruit characters so that a absence of an accurate monographic treat- wide range of variability is available for ment of the entire genus. The only com- exploitation by plant breeders. Valuable prehensive monograph was published by genes within the species include those con- Cogniaux (6) and was based entirely on ferring resistance to several diseases such as herbarium specimens. Cogniaux described powdery mildew (4), downy mildew (3), many species that were separated from one Alternaria blight (3), gummy stem blight another by minute differences poorly defined (32), watermelon mosaic virus 1 (35, 37), in keys. Systematic studies by taxonomists and cucumber mosaic virus (4, 35) and have reduced many of Cogniaux species to resistance to at least one insect (12, 13). synonymy and have removed a few species Despite great variability in Cticnwis mebo to other genera. and diligent search among its multitude of Since our own studies were initiated, parts cultivated, semi-feral, and feral forms from of the genus have been studied critically by all parts of the world, no plants have been Meeuse (23) and Jeffrey (14). Mecuse found with sharply defined, potent resist- collected and studied plants in their natural ance to crown blight, watermelon mosaic habitats in southern Africa and in experi- virus 2, Verticilliurn wilt, several nematodes, mental plantings used for Enslin & Rehm s (8) work on bitter principles. Meeuse (loc. and several insects. Screening studies for cit.) gives good descriptions of the species, nematode resistance by Winstead & Sasser their habitats, and ranges. Also there is a (39), Fassuliotis & Rau (10), and Fassulio- good key to 17 species of Cncninis found in southern Africa. Jeffrey (loc. cit.), an au- 1 Geneticists, Plant Science Research Division, thority on the family Cucurbitaceae, col- Agricultural Research Service, Charleston, South Carolina, and La Jolla, California. Submitted lected and studied plants in their natural for publication February 24, 1971. habitats. His treatise of the Cucurbitaceae 195 ( ()\O\Il( 110 1 \N)

TABLE I CUCUMIS SPECIES USED IN HYBRIDIZATION STUDIES

Chromo- some Entry Species No. (2n) Source Origin la C. sativus L. 14 Breeding lines United States lb C. hardwickii Boyle 14 P. I. 183,967 himalayan foothills, India 2 C. melo L. 24 Breeding lines United States 3 C. hIluIiJructIIs Stent. (Meeuse) South Africa 4a C. dioteri Cogn. 24 P. I. 282,444 (Rehm) Southwest Africa 41) C. sagittatus Peyr. 24 P. I. 282,441 (Rehm) Southwest Africa 5 C. rnetoiifcrus Naurl. 24 P. 1. 202,681 (Rehm) Northern Transvaal 6a C. anguria L. 24 P.1. 196,477 (Rehm) Brazil Oh C. anguria var. ion gipes P. I. 282,442 Southern Rhodesia (Hook. f.) Meeuse 24

7 C. dipsaceus Spach. 24 P. 1. 193,498 Ethiopia P. I. 282,440 (Rehm) Northern Cape Province, 8 C. africanus L. f. 24 Witfontein, Transvaal 9a C. leptodermis Schwcik. 24 P. I. 282,447 (Rehm) Orange Free State 91) C. myriocarpus Naud. 24 P. I. 282,449 Pretoria lOg C. prophetarum L. 24 P. I. 292,396 Israel 101) C. zeyheri Sond. 24 P. I. 282,450 (Rehm) South Africa ila C. ficifolius A. Rich. 48 P. I. 196,844 Ethiopia llb C. heptadactylus Naud. 48 P. I. 282, 446 (Rehm) Northern Cape Province

(in Flora of Tropical East Africa) contains tetraploid clones of C. melo and C. sativus a critical treatment and key to 13 species of cross-pollinated in all combinations. Shan- Cucomis native to tropical eastern Africa. mugasundaram et al. (28) reported a sue- Rosette Fernandes & A. Fernandes treatise cessful cross of C. inelo with C. pubescens. (11) has keys and descriptions in Portuguese This cross appears to be an intraspecific of seven species of Cucumi.s collected in rather than an interspecific cross. Angola. These studies in combination give Andrus & Fassuliotis (1) obtained 1:3 an excellent picture of the genus in areas interspecific crosses including eight with where species of Cucuin 1.5 are relatively Cucurois anguria, but pollinations with C. abundant. meio or C. sativus as a parent yielded no The first recorded attempt to cross Cucu- viable progeny. The present report confirms mis inebo with other species appears to have and extends that of Andrus & Fassuliotis been made by Naudin (24). He was un- (loc. cit.). Studies on morphology, phys- successful in obtaining a cross between C. iology, and compatibility are combined to meio and C. m uriocarpus. Koslov (16), aid a phlogenetic interpretation of available Vavilov (33), and Pangalo (25) apparently species of Cucuinis. reported artificial and natural hybrids within C. niebo. Whitaker & Davis (38) failed to Methods secure hybrids from crosses among the three cultivated species, C. angurlo, C. nselo, and Standard pollination techniques for Cucu- C. sativus. Batra (2) and Smith & Venkat mis (38) were used for crosses in green- Ram (30) were unable to cross C. mebo house and field. with C. sativus at the tetraploid as well as In addition to the standard techniques, the diploid level. In unreported studies some special techniques were used in at- performed at that time we also failed to tempts to secure crosses of Cucumis melo secure species hybrids from diploid and with other species: (1) fruit-setting hor-

DEAKTN ET AL.: INTERSPECIFIC 1IYI3111I)IZAFH) 197

lamlit ickii have In.. J. (IcuIuia .OIliUus, C. /U1Ii/tLUIsll. tlI(ir F In hrid, wd thu Ijtukurus to C. sparsely stiff-spined fruits. Size, shape, and color vary in different ctilti ars of C. sativils, and the spines may not persist. mones were applied to pistillate flower buds their parents and used as parents in con- at pollination; (2) chemical senescence in- trolled pollinations. Controlled pollinations hibitors were applied to pistillate flowers at were made to secure selfs, backcrosses to pollination; (3) mixtures of pollen from both parents, and outerosses to other species. several species were applied to stigmas of Pollen fertility was determined by effective- single flowers; (4) pistillate flowers were ness in pollinations and by counts of stained pollinated at anthesis and at 1 and 2 days pollen in aceto-carmine or lacto-phenol/ before anthesis; (5) F 1 , F0, and three-way fuchsin smears. interspecific hybrids were used as parents in reciprocal crosses with C. melo; (6) Materials tetraploid lines of C. inelo and some other Several species of Cucurnis exist as parts species were used in reciprocal crosses; and of the native flora in Africa, India, and (7) embryo culture was attempted with a neighboring lands. Until recently seeds of few minute embryos from several interspe- most of these Old World species were not cific crosses. Several of the wild species available to Western plant scientists. Seeds were used as parents in those crosses, but in of some species are still unavailable to us. Cucv- all cases, C. melo was used as one of the We obtained seeds of 16 species of parents. mis (Table I) through the United States Putative hybrids were compared with Department of Agriculture, Agricultural Re-

198 ECONOMIC( BOTANY

fr.. .3. The w liitu, subterranean fruits of C, huiuifructus are ornamented with net-like ridges. The oblate fruits average 35 mni long x 40 mm across.

yellow stripes (Fig. 1). Both species have seven pairs of chromosomes (Table I). 10011 B 2. Cultivars of the annual muskmelon, CMI C. nielo, bear young fruits with abundant, Fic. 2. i\laturc fruits of C. iiiclo ary greatly long, soft, fragile hairs that may persist. The in size, shape, color, aroma, and ornamentation. mature fruits vary greatly in size, shape, Soft hairs on the young ovaries may persist, or color, surface ornamentation, and other char- the mature fruits may be smooth, ribbed, or acters. Cultivars include cantaloup, honey- ornamented with corky net. Cultivars usually dew, casaba, Chinese cucumber, and other weigh in the range of 2 to 20 lbs., but wild types. Many are large and ellipsoidal (Fig. types may weigh as little as 10 grams, as in (b), 2), but fruits of the Indian cultivar Kakri from India, and its F1 hybrid with cultivar Georgia 47 (a). may measure 3 feet long X 3 inches wide. Most American cultivars are andromonoecious, but those of other lands usually are search Service, New Crops Research Branch, monoecious, and other sex forms occur. as Plant Introductions (P. I. numbers), and Young fruits of monoecious, annual, wild through the courtesy of A. D. J. Meeuse, C. melo from Africa and elsewhere also are formerly with the Division of Botany, Pre- ornamented with hairs. The small, usually toria, and S. Rehm, formerly at the Horti- ellipsoidal, mature fruits are often green cultural Research Station, Pretoria, Trans- with darker mottle or stripes, but bright yel- vaal, South Africa. low-orange and other colors occur. Original Fruits of the 16 species are illustrated in collections of wild and cultivated C. melo 11 figures. Fernandes & Fernandes (11), have 12 pairs of chromosomes (Table I), Meeuse (23), Jeffrey (14), and others have but 48-chromosome breeding stocks have given good technical descriptions of them. been derived from natural and coichicine- 1. Cultivars of the predominantly monoecious annual cucumber, induced mutants of muskmelons as well as Cucumis sativus, squashes (2, 27 fig. 3). bear young fruits with scattered short, sharp, stiff spines that may persist. The mature 3. Other wild species of Cucumis with fruits vary in size, shape, color, and surface hairy rather than spiny young fruits include markings. Many are large and elongate the monoecious annual C. huniifructus and (Fig. 1 and Table I entry la). the monoecious perennials C. dinteri and C. Fruits of the wild, monoecious, annual, sagittatus. These species also have 12 pairs bitter cucumber of India, C. hardwickii, also of chromosomes (Table I). bear scattered, short, sharp, stiff, often eva- The pedicels of pistillate flowers of C. nescent spines. The small, ellipsoidal (5 X himnmifructus elongate after fertilization and 4 cm) mature fruits are green with ivory or push the small, pointed pistil into the ground

DEAKIN ET AL.: INTERSPECIFIC IlYBIUDIZATION 199

ul C. no Ic. 1) ( al Fic. 4. Ovaries of wild, African C. ilintc,i (a) and C. agit1atua çh), likc tIiu that may persist on the brownish green or yellow mature fruits. The short, ellipsoidal fruits average 45 x 35 mm and 50 x 40 mm. before the carpels start to enlarge. The inedible, bitter forms occur (Meeuse, 23). cm fruit is sparsely covered with ivory-white, oblate fruit, 5 to 9 cm in diam- The 12 x 7 eter, has a rough, check-marked, waxy sur- large, very stout, stiff spines (Table I entry 5 and Fig. 5). The red-orange fruits have face (Fig. 3). Meeuse (22) has shown that this unique species is partly dependent for thin white flesh, green jelly-like pulp, and dissemination on the aardvark, which digs seeds covered with fine hairs. The monoe- up and eats the geocarpic fruits. cious, annual plants have 12 pairs of chro- 4. The nearly globose (3.5 cm), green, mosomes. C. dinteri (Fig. 4a) 6. Fruits of the sparsely short-spined, stripe-mottled fruits of C. anguria (see Meeuse, 23) turn yellow when fully edible gherkin of the West Indies, var. anguria (Fig. 6a), and the abundantly mature. The flowers in our cultures were gipes pleasantly aromatic. The slightly larger (4 long-spined, bitter C. anguria var. ion C. sagittatus (in Fernandes of Africa (Fig. 6b and see Meeuse, 23) are to 5 cm) fruits of to 5 cm in & Fernandes, 11, = C. angolensi-s Hook. f. ex usually 5 to 6 cm long and 3 diameter. Both monoecious annuals have 12 Cogn. in Meeuse, 23) turn pale yellow more pairs of chromosomes. readily (Fig. 4b). The flowers in our cul- C. Cucurnis species, 7. Fruits of the "dipsaceus gourd," tures, like those of most 5 cm, are were not aromatic. Both species have 12 dipsaceus, measuring about 7 x pairs of chromosomes. densely covered with long, thin, soft spines, 5. The African "Horned Cucumber," C. with each spine ending in a hyaline bristle metuliferus, is eaten in Africa but grown as (Fig. 7). The pale green fruits turn uniform an ornamental in the United States. Wild, yellow at maturity. This species is native

200 ECONOMIC BOTANY T

Fic. 5. The brownish ii -ii ottkcI, orange I nil ts of C. niel iiliferus, the African 1 Ion rd Cucuni- her, have unique, thick, fleshy spines. The long ellipsoidal fruits average 115 x 70 mm.

DEAKIN ET AL.: INTERSPECIFiC IIYB1IIDIZATION 201 to NE tropical Africa (Jeffrey, 14) but is ribs (Fig. 91)) Both species arc iliol occions annuals with 24 chromosomes. cultivated for ornament and is sometimes mm, adventive elsewhere. The monoecious, an- 10. All surfaces of the 44 >< 35 nual plants have 12 pairs of chromosomes. uniformly pale yellowish green fruits of C. are moderately well covered 8. The nearly cylindrical (8 x 4 cm), pro plictaruoi brownish, ivory flecked and striped fruits of with short, curved, soft spines (Fig. lOu C. afrtcanus are well covered with stout, Collections differ in number and length of blunt, conical spines (Fig. 8). Edible forms, spines, and some are faintly striped. The like those shown, and smaller, poisonous 52 x :37 mm, pale and dark green striped forms occur in southern Africa (Meeuse, fruits of C. zeylieri are similarly spineci on The monoecious, annual plants have all surfaces (Fig. 10b). Both species are 23) . monoeeious perennials with 24 chromosomes. 24 pairs of chromosomes. 50 X 35 mm, in- 9. The small, nearly spherical (20 >< 19 11. All surfaces of the mm) , pale greenish yellow, faintly striped distinctly pale and dark green striped fruits are sparsely ornamented with fruits of C. leptodermis are sparsely orna- of C. ficifolins mented with short, soft spines (Fig. 9a) very short, soft spines (Fig. ha) . The plants are monoecious. The thick-spined, The sharply ivory and brownish green mm), striped striped, ellipsoidal (25 x 22 mm) fruits of brownish green, oval (53 x 45 C. Iieptadactijlus resemble those of C. rntjriocarpli.S are densely spined on the fruits of

C. aogurPi \ ar. aoguria, and FIG. 6. Fruits of the pale yellow, cu1ti ated \Vct Indian Chcrkin, the pale green, wild African C. anguria var. loiigipe.s differ greatly in spine ornamentation. The short ellipsoidal fruits average 60 x 45 mm and 45 X 38 mm. MLAW

202 E(O\o\11( iuYrAN

.1:- I ,r I

Lfr

54 - v%4 --

I T1 c t 1I 2 3i 4i 5t 6! 71 91 WI flI

FIG. 7. The densely soft-spineci, apple green fruits of C. dipsaceus, from NE tropical Africa, make it useful as an ornamental plant. The ellipsoidal fruits average 70 x 45 mm.

C. africanus and C. niyriocarpus (Fig. lib); C. anguria var. longipes to C. anguria var. the linear leaflets are unique and the plants anguria. are dioecious. Both species are perennial Cucuni is harclwickii and C. sat ivus failed with 48 chromosomes. to set fruits, or set fruits with seeds lacking embryos from cross-pollinations with the 24- Results and 48-chromosome species (Table II). Cucumis nielo A. Primary crosses, sells, and backcrosses 2. All collections of wild C. melo and all 1. Cucumis sativus and C. hardwzcku cultivars of C. inelo cross readily with one Cucumis sativus and C. hardwickii, indig- another. More cross-pollinations were at- enous to India, are diploids with seven pairs tempted with C. rnelo than with other spe- of chromosomes. They set fruits with full cies. Fruits were set from several crosses complements of plump seeds from reciprocal with and without the aid of fruit-setting cross-pollinations. The hybrids were mostl y hormones. Interspecific pollinations, in all fertile but with slight sterilit y similar to that combinations, between diploid and tetra- found in wide crosses within C. web. Those ploid lines of Cucumis mebo and C. sativus results support Rehms statement 2 that mar- cultivars set some fruits, especially on C. phological characters and cucurbitacm con- sativus, without hormone aids. Such fruits tent fail to differentiate the two entities as contained ovules that were sometimes en- distinct species. Perhaps C. hardwickii bears largeci, but they did not contain embryos a relationship to C. sativus similar to that of visible to the naked eye. The cross C. nicbo X C. hardwickii gave similar results. 2 In personal correspondence to T. W. Whit- Pollinations on Cucuwi.s inebo with pollen aker 3.9.68. from C. dinteri, C. sagittatus (C. ango- o:3 OLAKIN er AL.: Ni LIiSiECIFi(1 HYB1JII)1ZAI ION

little indicatiou that C. Inch) could be en )ssed directl y in either direction with all y of those species. C nc urn is Ii urn ifruct i is The unique subterranean fruits of in is Ii ii in if met us resembled those of C. nicto more closely than did those of other species. The plants, also, were similar to those of C. nielo. However, the pistillate flowers of C. hi,inifrnctus are uniquely specialized to force the pistil into the soil after pollination. C. Iuiinifmnctns set fruits that failed to de- velop from cross-pollinations with pollen from C. mob, C. dinteri, and C ..agitlat 115 (Table 11) . It failed to set fruits from cross- pollinations with 12 other species. 4. Cucuniis dinteri and C. sagittatus Cucumis dinteri and C. sagittaffis yielded fertile hybrids from reciprocal cross-pollinations with each other (Table II). This agrees lie. 8. The 1 )luilt-spnled, nearly cylindrical, with S. Rehms work as reported by Meeuse brownish green-mottled and ivory-striped fruits (2:3, p. 65) and supports Fernandes & Fer- of C. africanus average 80 x 40 nun. nandes (11) opinion that the two entities are members of a single species. lensi) , or C. /iiiinifiitctus initiated fruit Cucurnis dinteri, used as it female, set development, but the fruits either failed to fruits with seeds lacking embryos or with mature or they matured with partly devel- minute embryos that failed to emerge from oped seeds. Visible embryos were absent sand plantings from crosses with C. inclo in the seeds, or they were small and failed and C. hnoufructus. It set fruits with non- to emerge from soil or sand plantings. Re- enlarged ovules or failed to set fruits from ciprocal crosses gave similar results. C uctinii.s inelo cross -pol Iii ationS with 12 other species. Cross-pollinations between set a single fruit that and the remaining 10 species usually failed Cucnniis sagittatus to set fruits without the aid of fruit-setting contained enlarged ovules lacking embryos C. zeylicri. It hormones. The fruits set with such aid from a flower pollinated b y contained nonenlarged ovules. There was set fruits with ovules lacking embryos from

A ilk,lillil 4 5 7

greenish yellow ,str iped h nits of FIG. 9. (a) The small, nearly spherical (21 X 20 HIm), pale C. leptodermis are sparsely ornamented with short, soft spines on the darker stripes. (h) The small, short-ellipsoidal, brownish green-ribbed, ivory-striped, and densely spined fruits of C. niyriocarpil.s average 25 x 22 mm.

204 ECONOMIC BOTANY

- 4 -

• B 3L

Ic. 10. (a) The ellipsoidal (44 X 35 min),palegreenish ii fruits of C. )wpIu In, out ui, be unculorons or indistinctly striped All surfaces bear curved, soft spines that vary in lengthand ,dtundancc in different collections. (1) The ellipsoidal (52 ,-, 37 tom), dirk and pus green-striped hulk of C. z i/lu ri turn s1lnw at hull maturity. All surfaces bear cured_ short, soft spines.

crosses with C. melo and C. prophetarom. fruits from reciprocal cross-pollinations with Cross-pollinations with seven species failed 5 other species and C. angurin var. iongipes to set fruits (Table II). (Table II). 5. CUCUThIS ntetulzferus 6. Coconuts aogoria and its variety ion gipes The uniquely thick-spined, monoecious, The cultivated variety aoguria of Cucumis mesophytic, diploid cultivar Coconuts nletu- anguria crossed readily with its wild variety liferus, "Horned Cucumber" failed to set ion gipes. The hybrids were vigorous and

DEAKIN ET AL.: iNTERSPECIFIC HYBRIDIZATION 205

/iciJolius are sparsely Fm. 11. (a) The ellipsoidal (50 >( 35 min), striped green fruits of C. ornamented with very short, soft spines. (b) The ellipsoidal (53 X 45 mm), striped, brownish green C. africanus and fruits of C. heptadact plus bear short, thick spines intermediate between those of C. inijriocarpuS.

C. heptadac- fruitful (Table II). They produced flowers from cross-pollinations with with 97% stainable pollen and set fruits with tyios, C. dinteri, C. mettdiferus, C. hard- abundant seeds from selfed flowers. Our wickii, and C. sativus. results agree with those of Meeuse (22) and The successful crosses yielded F 1 hybrids support his view that the two entities are that ranged from partly fertile ones with Cucumis africanus and C. anguria to sterile varieties of a single species. C. ficifolius and Cuctimis anguria varieties could be crossed or nearly sterile ones with with C. clipsaceus, C. africanus, C. leptoder- C. zeyheri. Most male flowers on plants mis, C. inyriocarpus, C. ficifolius, C. pro- from the latter two crosses aborted, but phetarum, and C. zeyheri, and they yielded stainable pollen from F 1 flowers that did hybrids. Stainable pollen reach anthesis averaged 9% in F C. dip- partly fertile F1 C. fici- ranged from 14% in the F1 C. prophetartim sacens >< C. zeyheni and 4% in F 1 17o in the F5 C. zeyheri folius >< C. dipsaceus. The, greater fertility • C. anguria to 24 was • C. anguria. Backcrosses to the parental of the hybrid C. dipsaceus>< C. zeyheri species were either unsuccessful or yielded indicated, also, in fruits set from backcrossed only a few plump seeds. C. anguria vars. flowers. Such fruits averaged 39 and 38 ion gipes failed to yield viable plump seeds, respectively, from pollinations anguria and C. zeyheni. seeds from cross-pollinations with C. hepta- with C. dipsaceus and with duct plus, C. dint en, C. melo, C. metuliferus, Fruits set on the F 1 C. ficifolius >< C. dip- C. hardwickii, and C. sativus. saceus averaged only one partly plump seed from backcrosses to either parental species. Cucumis dipsaceus Fertility was partly restored in plants from In addition to the crosses with Cucumis the backcross (Cucumis dipsaceus X C. zey- anguria mentioned above, C. dipsacens was hen) x C. zeyheni. Stainable pollen in four successfully crossed with C. of ricanus, C. plants ranged from 20% to 76%. ficifoiiu.s, and C. zeyheni. It set fruits lacking Cue umis africanus C. leptoder- plump seeds from crosses with C. mis, C. myriocarpn.s, C. melo, and C. pro- Cucuniis afnicanus crossed readily with phetaruns. C. dipsaceus failed to set fruits anguria vars. anguria and longipes; also with 206 ECONOMIC BOTANY

TABLE II SELF- AND CB0SSCOMPATIBILITIESa AMONG CucuMis SPECIES. SPECIES PERFORMANCE AS FEMALE PARENTS ARE INDICATED IN HORIZONTAL Bows; THEIR PERFORMANCES AS MALE PARENTS ARE INDICATED IN VERTICAL COLUMNS.

0 a a a a C 1 CI a C - :•- .2 f - a .21

Species 3 C. sativus a rn 14 5 4 1 0 1 1 0 0 0 0 0 0 0 0 0 0 C. hardwickii a rn/a 14 4 5 0 0 0 0 0 0 0 0 0 0 0 C.otelo arn/a24 0 0 5 1 1 1 0 0 0 0 0 0 0 0 0 0 0 C. Isuniifrnctus a rn 0 0 1 5 1 1 0 0 0 0 0 0 0 0 0 () C. dinteri p m 24 0 0 1 1 5 4 1 1 1 0 0 0 1 0 0 0 C. sagittatus p rn 24 1 4 5 0 0 0 0 0 0 1 1 0 C. metuliferus a ni 24 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 C. anguria a rn 24 0 1 5 5 3 3 3 3 0 2" var. longipes a m 24 0 0 0 0 0 5 5 3 2 3 0 C. dipsacens a m 24 0 0 1 0 0 3 0 5 3 1 0 2 0 0 C. africanus a m 24 0 0 0 0 0 0 0 3 3 3 5 3 3 0 0 C. leptodernus a rn 24 0 5 5 C. myriocarpis a m 24 0 0 1 5 5 1 C. prophetarun p m 24 0 1 1 3 1 3 3 5 2 C. zeyheri p m 24 1 3 2 1 3 2 3 5 2" C. ficifolius p m 48 0 0 0 2 2 2" 5 C. heptadaci pins p d 48 0 0 0 0 0 0 0 0 0 0 1 5 Classes of cross-compatibility are indicated numerically: 5, fully fertile progeny; 4, progeny moderately self-fertile; 3, progeny sparingly self-fertile; 2, progeny self-sterile but sparingly fertile in crosses; 1, fruit set but no progeny; 0, no fruits set. Blanks, pollinations too few to certify compatibility status. a, annual; p, perennial. a, andromonoecious; d, dioecious; m, Inonoeciolls. 1 Triploicl offspring were ob-served.

C. dipsaceus, C. leptodermis, C. nlyriocar- Stainable pollen ranged from 17% to 34% pus, C. pro phetaruin, C. zeyheri, and C. in seven species hybrids. The backcross to ficifolius but not with the remaining eight C. anguria using the F1 as a male parent species (Table II). Some of the crosses demonstrated viable pollen in the C. angurki could be made more easily in one direction. X C. africanus hybrid. Seeded fruits from For example, C. africanus crossed easily and naturally pollinated flowers indicated that yielded vigorous, normal green, moderately most or all of the F1 hybrids produced func- prolific F1 hybrids from crosses with C. tional pollen. angurw used as a female parent. The recip- Most of the F1 hybrids backcrossed to rocal cross yielded fewer viable seeds, and their respective parents yielded 30 to 50 the few resulting seedlings were initially plump seeds in a fruit. Curiously, the hybrid weak and light green in color. Plant color from the wild CUCUII1i.S anguria var. longipes improved in the field, however, and the X C. africanus yielded only about 10010 to mature plants were comparable in vigor and 15% as many seeds in a fruit as did the prolificacy with those of the reciprocal. hybrid from the cultivated C. anguria var. Some crosses were successful in only one anguria x C. africarius. direction. Most of the backcross plants were robust Male flowers aborted on F 1 hybrids from and fruitful, but they varied greatly in size, some of the successful crosses. However, appearance, and percentages of stainable some mature flowers with nearly normal pollen. Occasional dwarf-like B 1 plants anthers occurred on most of the hybrids. occurred. Percentages of stainable pollen 207 DEARIN ET AL.: INTERSPECIFIC HYBRIDIZATION

in morphological characteristics to C. ficif 0- ranged from 29% to 9017o in individual plants, bins, and the tetraploid accession, labeled and averages from five representative B1 was finally identified as populations ranged from 49% to 68%. C. rnembranifolius, C. ficifolius. The other accession, labeled 9. CncwnLs leptodermi.s and C. inyriocarplis C. pustu/atus, was a diploid with some C. zeyheri Cucuniis leptoclernits crossed readily with characters in common with both and C. ficifolins. These findings indicate C. iii tjriocarpus and yielded vigorous, fully C. hybrids. Our results agree with that it may be either a diploid race of fertile F1 or that it represents a link between those of Meeuse (23), who stated: "C. lep- f icifolius C. C. ficifolius and C. zeyhcri. todermis hybridizes quite easily with yielded self-sterile hy- apparently not only after arti- Cucuinis ficifolius inyriOCarpus, brids froni crosses with C. africanus, C. ficial cross-pollination, but also in nature. C. dipsaceus (Table The two entities yielded similar results in angtiria, C. zeyheri, and attempted crosses, yielding weakly fertile II). The last three were sparingly fertile in C. africanus and C. anguria crosses. Chromosome counts indicated that hybrids with the first three species hybrids were triploid. but none with eight other species (Table and yielded weakly fertile Cross-pollinations between C. ficifolius II). C. myriocarpus the first five species in Table II failed to set hybrids also from crosses with C. pro phe- tarUm and C. zeyheri. Our results support fruits. Meeuses suggestion that ". . . perhaps C. 12. Cucumis heptadactylus leptodermis is not more than a variety or a Cu- C. myriocarpus" (loc. cit.). The dioecious, xerophytic, tetraploid subspecies of cum is heptadactylus has uniquely linear 10. Cucuniis pro phewrum and C. zeyheri leaflets. It set fruits lacking viable seeds C. myriocar PUS. Cucunsis prophetarum x C. zeyheri from cross-pollinations with yielded vigorous F 1 hybrids that were It failed to set fruits from reciprocal cross- weakly fertile (Table II). Male flowers pollinations with 10 other species (Table II). yielded an average of 37 0/c stainable pollen. B. Outcrosses to a third species One or both species yielded weakly or very Nicotiana, weakly fertile hybrids from crosses that were Work by Kehr & Smith (15) in attempted with C. anguria, C. dipsaceus, C. and by Wall & York (34) and Whitaker & Cucurbita demonstrated that africanus, and C. myriocarpus. Both species Bemis (36) in failed to set fruit or set fruits lacking viable interspecific incompatibilities can be over- seeds from attempted crosses with the first conic in some plant genera by using hybrids from compatible species crosses as one or seven species in Table II. The breeding results failed to furnish both parents. Thus the gerrnplasm from two strong evidence to support Jeffreys state- species that refuse to cross can be combined C. zeyheri is a subspecies in a single individual by using a third species ment (14) that compatible with both of the others as a of C. pro phetarum. We believe that for the present C. zeyheri should be retained as a "bridging" species. distinct species separate from C. pro pheta- Repeated attempts were made to break the compatibility barrier to the Cucumis rum because they yielded weakly fertile melo >< C. anguria cross. No species was hybrids from crosses with one another. In C. addition, their distributions differ; C. pro found that was cross-compatible with a native of eastern tropical inebo, but several were cross-compatible with phetarum is Accordingly, numerous cross- C. zeyheri is a native of southern C. anguria. Africa, but pollinations were made between C. mebo temperate Africa. and the several F 1 hybrids described above. 11. Cucumis ficifolius Most of the cross-pollinated flowers failed According to Shimotsuma (29), the mo- to set; the few that did set produced fruits with nonenlarged ovules, or with partly en- noecious, perennial Cucumi-s ficifolius is tetraploid. In addition to several of our larged ovules lacking embryos. To broaden the germplasrn base within accessions labeled C. ficifolius, two lines and C. pustulalus the cross-compatible group, pollinations were labeled C. membranifolius species hybrids and a respectively were studied. Both were similar made between F 1

208 ECONOMIC BOTANY

third species partly cross-fertile with both cytogenetic, phylogenetic, and physiological parents of the F1 hybrid. The close relationship between C. study. They may also have greater potential anguria and the other seven than the parents in crosses with Species in species in the anguria group was borne out the Cucuinis melo group at the diploid or in hybrids from such three-way crosses. The tetraploid level. F1 hybrid from the cross C. clipsaceiis x C. The successful cross of anguria Cucumi.s hepta- was outcrossed to C. africanus, C. clactylus with the F1 inyriocarpus, and C. hybrid from the cross zeyheri, yielding hy- C. anguria x C. africanus adds that species I brids with a full genome of one species and to the group related to C. anguria. partial genomes of the other two. Small The F1 Cucumis clipsaceus x C. zeyheri (eight-plant) populations varied in stainable was outcrossed to C. africanu.s and to C. pollen percentages from 1% to 35%. Fur- anguria. The first three-way cross yielded thermore, nearly all of the plants set fruits seven plants with 8% to 40% stainable pol- with some seeds from natural pollinations, len. The second yielded five plants with and some of them were surprisingly prolific 3% to 187o stainable pollen. All of the and fertile. plants produced seed. The F1 hybrid from the cross Cucnmi.s The results indicate that an admixture of anguria x C. africanus was outcrossed to C. genes from a third species within the anguria nn/riocarpus, yielding two three-way hybrids group did not destroy the ability of the with 36% and 761/c stainable pollen. Both plants to produce stainable pollen and viable plants produced numerous seeded fruits seeds. In fact, the greater variability in all from naturally pollinated flowers. characters in the three-way hybrids and A promising indication that cross-incom- their surprisingly high, partial fertility sug- patibilities can eventually be broken in Cu- cumis gest that they may be useful in securing was secured in a successful three-way hybrids with species outside this large group cross from the F1 (C. anguria x C. africanus) of partly cross-fertile species. x C. heptadactylus. That cross yielded robust, prolific plants with 47% to 81% stain- Discussion able pollen. The fruits resembled those of C. heptadactylus in shape, color pattern, and The interbreeding results indicate that several spiny-fruited wild African species of spine size; in other characters the three-way Cucumjs hybrids resembled C. anguria more than the are related to the West Indian other parents. The fertility of these triple Gherkin, Cucumis anguria var. anguria. Cu- hybrids, which was much greater than either cumis angiiria is economically the most im- that of the F1 hybrid or that of C. hepta- portant member of this diverse group of dactylus itself, is impressive for several naturally or artificially interbreeding species. reasons: (1) C. heptadactylus failed to cross These species can be termed, collectively, directly with either of the other two species; the anguria group for convenience. The anguria (2) C. heptadactylus is a perennial, the group varies greatly in mor- other two species and their F1 hybrid are phology, ecology, sex differentiation, and annuals (Meeuse, 23); (3) C. heptadactylu.s chromosome number. It includes (1) the cultivated C. anguria is dioecious, the others monoecious; (4) C. var. anguria and the heptadactylus is a tetraploid with 48 chro- three mesophytic, bushland-inhabiting, mosomes, the others are diploids with climbing or trailing, monoecious, diploid (2n 24 = 24), somatic chromosomes (Shimotsuma, 28). slightly cross-fertile, annual herbs C. africanus, C. anguria var. longipes, and High fertility in each of the trihybrids C. dipsaceus may indicate that they have resulted from (Figs. 6, 7, and 8); (2) the two xerophytic, trailing, monoecious, diploid fertilization of unreduced megagametes. (2n = 24), interfertile, annual herbs C. Unreduced microgametes are frequently leptodermi.s and C. inyriocarpus observed in pollen mounts from the F1 (Fig. 9); (3) the two xerophytic, prostrate, monoecious, hybrids. If that hypothesis is correct, the diploid (2n = 24), moderately cross-fertile, trihybrids would contain a full genome from perennial herbs C. pro phetaruni and C. zey- each of the three parents. They furnish an hen (Fig. 10); (4) the xerophytic, trailing, excellent opportunity for combined genetic, monoecious, tetraploid (2n = 48) perennial

DEAKIN ET AL.: INTERSPECIFIC HYBRIDIZATION 209

behavior of the pistillate flower with its tiny herb C. ficifolius (Fig. 11); and (5) the xerophytic, prostrate, dioecious, tetraploid ovary, which delays its development after (2n 48) perennial herb C. lieptaclactylus fertilization, and its unique pedicel that resumes growth after fertilization and forces (Fig. 11). No associations as close as those among the ovary into the soil, present obstacles to group were found in cross-fertilization not found in any of the members of the anguria therefore, the other species studied. Morphological other species. C. humifructus, also remains unique with undetermined characters of Cucumis metuliferus (Fig. 5), especially its "horned" fruits and hairy seeds, affinities in the genus. suggest that it is unique. The breeding The studies reported here have demonstrated that several wild species of Cucumis results confirmed this view; no fruits were C. an.gnria, set from cross-pollinations with C. metuli- from Africa can be crossed with the West Indian Gherkin. Such crosses com- ferns as either the male or female parent. Cucuniis metu/iferus is therefore set apart bining germplasm from two, three, or perhaps several species furnish potential parents with no close relatives among the other C. me/a. Those species.: for use in crosses with Similarly, the chromosome complement of crosses are desirable in order to transfer Cucumis sativu.s and C. harclwickü ( seven resistance to nematodes and other parasites from C. anguria into C. me/a. Attempts to pairs compared with 12 and 24 pairs for C. me/a with C. other species), their harsh foliage and stiff- make such crosses using spined fruits (Fig. 1), and the natural dis- anguria and its hybrids have failed to yield tribution of the wild C. hardwickii in India, seeds with visible embryos. indicate that they are not closely related to In the search for a wild species that can Cucumis inc/a to break the the cultivated C. me/a or C. anguria and he crossed with incompatibility barrier, C. sagittatus, its close their wild relatives from Africa. Breeding C. huinifrnctns failed results confirm the distant relationship. All relative C. thnteri, and crosses of C. hardwickii and of C. sativus to yield viable hybrids from crosses with C. me/a. Theyalso failed to cross with each with other species either failed to set fruits species. It or set fruits with ovules Tacking visible con- other or with other Cucumis seems that the greatest likelihood of success tents. All of the species discussed above have in achieving the ultimate goal can be ex- spined ovaries. The remaining entities, C. pected from work with little-known but and C. presently unavailable relatives of C. me/a, me/a, C. humifructus, C. clinteri, or with wild forms of sagittatos (Figs. 2, 3, and 4), lack spines on such as C. sac /euxii,4 reported by Meeuse (23) to occur the ovaries. Instead, the ovaries are orna- C. me/a, mented with fragile hairs that often dis- in Africa. appear by the time the fruits mature. The Conclusions perennial C. thnten and C. sagittatns resemble one another and are interfertile. A. The genus Cucumis and its close rela- They bear little resemblance to either C. tives in the family Cucurbitaceae have long htnnifrnctus or C. melo, and crosses have supported a disorganized tangle of misinfor- failed to yield viable progeny. The closeness mation resulting from incorrect identification of their relationship to those species remains of plant cultures. Several disease resistance in question. surveys and other studies on exotic species The plants, seeds, and fruits of Cucurnis of cucurhits have reported information on lInmifruCtUs resemble those of C. me/a more unverified and often misnamed cultures, but closely than do those of any other species during the past decade many identity errors studied. However, the curious structure and in Cucnmis have been corrected by critical inquiry. Since this was written, J . D. Norton reported B. The data from cross-fertility tests are a successful cross between C. oietuliferus and compatible with data from studies on mar- feral C. nsela (P. I. 140471) in the 1969 Bien- nial Report of Vegetable Breeding in the South- personal correspondence by ern United States, Hawaii, and Puerto Rico, p. Mentioned in 53. C. Jeffrey. r

210 ECONO\ft(; BOTANY

phologv, cytogenetics, phytogeography, and riers. Spine differentiation occurs physiology (cucurbitacins). They indicate: occurs within as well as between interfertile entities. I. C. anguria var. Ion gipex is closely related to C. anguria var. anguria and may be its Literature Cited progenitor. 1. Andrus, C. F. & C. Fassuliotis. 1965. 2. C. leptoderm is and C. myriocarpus are Crosses among Cucurnis species. Veg. related as varieties or subspecies rather Improvement Newsletter 7: 3. than as distinct species. 2. Batra, S. 1952. Induced tetraploicly in 3. C. zeyheri is no closer than a subspecies muskmelons Jour. I-Iered. 43: 141-148. 3. Bohn, C. \V., C. F. Andrus, & R. T. Correa. to C. prophetarurn, and it may be a distinct species. 1969. Cooperative muskmelon breeding 4. C. program in Texas, 1955-67: new rating dinteri and C. sagittatus are related scales and index selection facilitate de- as varieties or subspecies rather than as velopment of disease-resistant cultivars distinct species. adapted to different geographical areas. 5. C. hardwjc/cjj and C. sativus are related U.S. Dept. Agr. Tech. Bul. 1405. as varieties rather than as distinct species. 4. & T. W. Whitaker. 1964. Ge- netics of resistance to powdery mildew C. The cross-fertility studies indicate that race 2 in muskmelon. Phytopathology Cncizmic 54: 587-591. species can be tentatively orga- 5. Brown, C. B., J iiized into four cross-sterile groups: . R. Deakin, & M. B. Wood. 1969. Identification of Cucurnjs species I. Eight spiny-fruited, African species or by paper chromatography of flavonoids. varieties of Cucwnis are closely related Jour. Amer. Soc. Hort. Sd. 94: 231-234. to C. anguria var. 6. Cogniaux, A. & M. Harms. 1924. Cucur- anguria, yielding partly bitaceae fertile hybrids with it. Cucurbiteae_._Cucumerjnae Das Pflanzenrcicl- (A Engler) 88(IV, 275, 2. C. inetuliferus is not closely related to 11) 1-246. any other species used in these studies. 7. Corley, W. L. 1966. Some preliminary 3. C...ativus, including many cultivated evaluations of Cucuinig plant introduc- forms and the spin y-fruited, seven-chro- tions. Ga. Agr. Expt. Sta. Bul. N. S. 179, mosome C. harclwjckjj from India, is not 58 pp. closely related to any of the spiny-fruited, 8. Enslin, P. B. & S. Rehm. 1958. The dis- 12-chromosome species from Africa or tribution and biogenesis of the cucurbita- ems in relation to the taxonomy of the their cultivated relative, C. anguria var. anguria. Cncurbjtaceae Proc. Linn. Soc. London 169: 230-238. 4. The three African species with young 9. Fassulintis, G. 1967. Species of Cucnniis fruits hairy and lacking operculate spines resistant to the root knot neinatode, Me- (C. humifructus, C. melo and its culti- loidogyne incognita acrita. Plant Dis. vated forms, and C. sagittatus including Rptr. 51: 720-723. C. dinteri) are more closely related to 10. & C. T. Ran. 1963. Evaluation one another than they are to any of the Of Cucunijs spp. for resistance to the other species studied. cotton root-knot nematode, Meloidogyne incognita acrita. Plant Dis. Rptr. 47: 809. D. Crosses of C. inelo with C. sagittatus yield partly developed seeds. Work with 11. Fernandes, R. & A. Fernandes. 1962. Con- additional collections of those species and trihulcao Para o conJiiecjmento das Cu- their close relatives may yield a "bridging curbitaceae de Angola. Mein. Junta In- vest. Ultrani. 2nd Ser. 34: 29-150. species" or form that will permit transfer 12. of genes from other species into Ivanoff, S. S. 1945. A seedling method C. nielo. for testing aphid resistance, and its ap- Embryo culture techniques may be useful in plication to breeding and inheritance culturing the minute embryos observed in studies in cucurbits and other plants. these crosses. Jour. Hered. 36: 357-361. 1:3. E. The abundance and specific structure 1957. The honiegarden canta- loupe, a variety with combined resistance of soft spines on fruits in the angnria group appear to be unrelated to cross-fertility bar- to downy mildew, powdery mildew, and aphids. Phytopathology 47: 552-556.

4 211 DEAKJN E1 AL.: INTERSPECiFiC HYBRIDIZATION

But. (;en. and Plant Breeding 23: 397- 14. Jeffrey, C. 1967. Flora of tropical east Africa. Cucurbitaceae. Crown agents for 442. 27. Pearson, 0. II., Richard Hopp, & C. W. oversea governments and administrations. Bohn. 1951. Notes on species crsses London. 157 pp. in Cocnrbita. Amer. Soc. llort. Sci. lroc. 15. Kehr, A. E. & H. H. Smith. 1952. Mul- tiple genome relationships in Nicotiana. 57: 310-322. Cornell Univ. Agr. Expt. Sta. Mciii. No. 28. Shanmugasundaram, S., B. W. X. Ponrialya, P. Chandrasekaran, & V. S. Raman. 311: 1-19. 1964. Studies on interspecific hybrids 16. Koslov, F. 1925. On species hybridiza- Madras Agr. tion in melons and cucurbits. Bul. Appl. in Cucumis and Cncurbita. Bot. and Plant Breeding, Leningrad 14 Jour. 51: 361. 29. Shimotsuma, A. 1965. Chromosome stud- (2): 71-78. species. Selken Knzuchov, Z. A. 1930. Karyological in- ies of some Cucurnis 17. Ziho Rept. Kihara Inst. Biol. Res. 17: vestigations of the genus Cucomis. Bul. Appi. But. Ccii. and Plant Breeding 23: 11-16. 30. Smith, P. G. & B. R. Venkat Ram. 1954. 357-365. Interspecific hybridization between musk- Leppik, E. 1966. Searching gene centers 18. melon and cucumber. Jour. llered. 45: of the genus Cucusnis through host-para- site relationship. Euphytica 15: 323- 24. 31. Sowell, Grover, Jr., S. W. Braverman, S. M. 328. Dietz et al. 1964. A summary of re- 19. 19661). Report of field observa- ports on the resistance of plant introductions and greenhouse testings on promis- tions to diseases, insects and nematodes. spp. at the NC-7 Regional ing Cucuinis Cucumis nielo. U.S. Dept. Agr. New Plant Introduction Station, Ames, Iowa, Crops Res. Branch, Beltsville, Md., mini- 1958-1964. Extracted from Plant Intro- eographed pamphlet. pp. 1-7. duction Investigation Papers No. 4, pp. 32. Prasad Krishna, & J. D. Norton. 1-3. 1966. Resistance of Cucunsis melo in- 20 1967. Relative resistance of Co- troductions to Mycospliaerella citrullina. coons introductions to disease and in- Plant Dis. Rptr. 50: 661-663. sects. Adv. Frontiers of Plant Sciences 33. Vavilov, N. 1925. Inter-genetic hybrids 19: 43-50. of melons, watermelons and squashes. 21. G. Sowell, Jr., S. W. Braverman Bul. Appl. Bot. Gen. and Plant Breeding et al. 1964. A summary of reports on 14: 3-35. the resistance of plant introductions to 34. Wall, J. R. & T. L. York. 1960. Genetic diseases, insects, and nematodes. Cucu- diversity as an aid to interspecific hy- mis sativus and Cuctonis spp. U.S. Dept. bridization in Phaseolus and Cucurbita. Agr. North Central Regional Plant In- Amer. Soc. Hort. Sci. Proc. 75: 419-428. troduction Station, Ames, Iowa, mimeo- 35. Webb, R. F. & C. W. Bohn. 1962. Re- graphed. 28 pp. sistance to cucurbit viruses in Cncumis 22. Meeuse, A. D. J . 1958. The possible melo. Phytopathology 52: 1221. (Abst.) origin of Cucnmis anguria L. Blumea 36. Whitaker, T. W. & W. P. Bemis. 1965. Suppl. IV (H. J. Lam Job. Vol.): 196- Evolution in the genus Cucurbita. Evo- 204. lution 18: 553-559. 23. 1962, The Cucurbitaceae of south- 37 & G. W. Bohn. 1954. Mosaic ern Africa. Bothalia 8: 1-111. reaction and geographic origin of acces- L. Plant Dis. 24. Naudin, C. 1859. Revue des Cucurbita- sions of Cucuniis melo cées cultivées an Museum en 1859. Ann. Rptr. 38: 838-840. Sci. Nat. sér 4 Bot. 12: 79-164. 38 - & G. N. Davis. 1962. The cucurbits, botany, cultivation and utilization. 25. Pangalo, K. I. 1930. Wild melons. Bul. App!. Bot. Gen. and Plant Breeding 23: Leonard Hill Books Ltd., London. 250 228-252. pp. illus. 19301). Critical survey of time 39 Winstead, N. N. & J. N. Sasser. 1956. 26. Reaction of cucumber varieties to five principal literature on the systematics, (Meloiclogyne spp.). geography and origin of cultivated and, root-knot nematodes 272-275. partly, wild growing melons. Bul. Appl. Plant. Dis. Rptr. 40: