J. AMER. SOC. HORT. SCI. 117(5):856-862. 1992 Breeding Strawberries at the Decaploid Level Hamid Ahmadi and Royce S. Bringhurst1 Pomology Department, University of California, Davis, CA 95616 Additional index words. Fragaria, unreduced gametes, day neutral, chromosome number Abstract. Two groups of Fragaria decaploid (2n = 70, x = 7) breeding populations were studied. The first was derived from pentaploid (2n = 35) and hexaploid (2n = 42) natural or synethetic interspecific hybrids between octoploid (2n = 56) F. chiloensis (L.) Duch. or F. virginiana Duch. both from California, and various Fragaria diploids (2n = 14). Their chromosome number was doubled with colchicine or through the naturally generated unreduced gametes. They were selfed repetitively, intercrossed, and open pollinated. Gametic viability of the her- maphroditic and female decaploid hybrids exceeded 50%. The hybrids exhibited heterosis for runner production and vegetative vigor. Fragaria chiloensis bred for large fruit and desirable fruit qualities, and, in combination with diploids F. vesca L. and F. viridis Duch., resulted in hybrids that produced a single early spring crop and prolific runner production throughout the summer. Fragaria virginiana L. derivatives were characterized by high pollen fertility, and by day neutrality (photo-insensitivity). Together, they may contribute genes for adaptation to various regions and climates of the world and for pest and disease resistance. The second and most important group of decaploids involved here were those derived from hybrids between day-neutral octoploid cultivars (F. ×ananassa) crossed to F. vesca or F. viridis. This group of decaploids combined the genomes of the best octoploid cultivars with those of the above diploid species: facilitating the incorporation of genes responsible for high yield, day neutrality, and excellent fruit quality into the decaploid strawberries. Polyploidization is the principal evolutionary force involved (1934) and Federova (1934) subsequently reported pentaploid in the development of many of the cultivated fruit crops, in- seedlings via unreduced gametes from F. moschata Duch. (2n cluding: plums (Prunus domestica L.), sour cherries (P. cerasus = 42) x F. nipponicaa Mak. (2n = 14) and F. vesca x F. L.) blackberries (Rubus spp.), highbush (Vaccinium corym- moschata, respectively. Scott (1951) obtained six fully fertile, bosum L.) and rabbiteye blueberries (V. ashei Reade), straw- short-day decaploid offspring from a hexaploid cultivar x tetra- berries and kiwifruit [Actinidia deliciosa (A. Chev.)], all of ploid ‘Alpine’ F. vesca hybrids open pollinated to octoploid which are allo- and/or auto-polyploids (Darrow, 1966; Janick cultivars. Bauer and Grubler (1962), Bauer and Bauer (1979), and Moore, 1975; Jennings, 1988; Warrington and Weston, MacIntyre and Gooding (1977), and Spiegler et al. (1986) used 1990). Some of these crops, such as octoploid strawberries, the same technique to obtain somewhat improved decaploid short- hexa- and heptaploid hybrids of blackberries and raspberries, day strawberries. Bringhurst and Khan (1963) and Bringhurst tetra- and hexaploid blueberries, polyploid currants, and kiwi- and Senanayake (1966) discovered vigorous sterile to semi-fer- fruit, were domesticated recently and are indeed new crops. tile natural pentaploid and hexaploid hybrids in overlapping For species differing in ploidy levels or with semi-incompat- populations of F. chiloensis and F. vesca in California. Bring- ible mating barriers and adaptive ecological differentiation, ad- hurst (1990), also obtained a semi-fertile hybrid of F. iinumae ditional polyploidization may be the most rapid and efficient Mak. with F. virginiana. procedure for using their germplasm to generate new wild forms In the investigation reported here, semi-fertile pentaploids or cultivars with improved adaptation. Amphiploid blackberry- and fully fertile colchicine or naturally derived decaploids were raspberry hybrids (Jennings, 1988) and natural decaploid deriv- obtained by combining genomes from wild populations of dip- atives of penta-, hexa-, and enneaploid hybrids of overlapping loid F. vesca including perpetual-flowering ‘Alpine’ and F. vir- populations of F. chiloensis and F. vesca in California (Bring- idis from Europe, with those of the several octoploid species, hurst and Gill, 1970; Bringhurst and Khan, 1963; Senanayake including F. ×ananassa day-neutral cultivars. Horticultural and Bringhurst, 1967) are examples of the above process. The characteristics such as yield, fruit size, and fruit quality were functioning of unreduced mega- and microspores from the semi- evaluated in their offspring. The objectives of this study were sterile to semi-fertile interspecific hybrids is the mechanism most to: 1) assess the possibility of obtaining fertile decaploid clones likely to advance this evolutionary process in nature. In addi- with sufficient desirable traits to form the basis for a breeding tion, functional unreduced gametes may expedite the transfer of program at the decaploid level, using genes from various diploid desirable genes from lower ploidy levels of wild or commercial species; 2) compare the efficiency of breeding procedures lead- progenitors to higher ploidy levels of fruit crops such as blue- ing to diverse decaploid populations; and 3) describe and eval- berries, blackberries, and strawberries (Jennings, 1988; Vorsa uate the horticultural attributes of selected decaploid strawberries. and Ballington, 1991). Many investigators have employed unreduced gametes to pro- Materials and Methods duce new synthetic clones of strawberries at various ploidy lev- els. Yarnell (1931) reported the synthesis of decaploid and The origin and fertility of decaploid and other ploidy levels enneaploid strawberry seedlings from semi-sterile pentaploid F. of Fragaria resulting from various hybridizations are shown in vesca × F. chiloensis hybrids via unreduced gametes. Lilianfeld Table 1. I) 8x x 2x ® 5x (colchicine doubled) ® 10x. One of the most useful octoploids was a wild clone of F. virginiana ssp. platyp- Received for publication 13 Sept. 1991. Accepted for publication 20 Apr. 1992. etala Rydb. from Luther Pass, Calif. (designated as ‘Luther’, We thank Carole Meredith and Fredrick Bliss for their critical reading and helpful comments The cost of publishing this paper was defrayed in part by the Table 2). ‘Luther’ was hybridized with F. viridis (designated payment of page charges. Under postal regulations, this paper therefore must progeny 83.242). The chromosomes of the hybrids were counted be hereby marked advertisement solely to indicate this fact. after the method of Bringhurst and Gill (1970), and the genome 1To whom reprint requests should be addressed. of the most vigorous seedling was synthetically doubled with 856 J. Amer. Soc. Hort. Sci. 117(5):856-862. 1992. The above clones and a 10x selfed derivative of CA 1349 (des- ignated selection 67.200-2) were selfed or hybridized to 83.242- 1 and their seedlings were evaluated in the field near Davis, Calif., in 1986. Their ploidy level, the relative success of the hybridization, and the viability of their gametes are noted in Table 1 (designated Ia-Id, Ih, and Ii). The designation system of Univ. of California, Davis, straw- berry accession or pedigree numbers are used throughout this manuscript because some of the clones involved are preserved at the National Clonal Germplasm Repository, Corvallis, Ore., and may be accessed through these numbers. II) 8x x 2x (unreduced) ® 6x (unreduced) x 8x ® 10x. A natural female hexaploid of F. chiloensis and F. vesca (desig- nated CA 1327) reported by Bringhurst and Senanayake (1966) was hybridized to ‘Aliso’ and two of their decaploid hybrids that resulted from unreduced hexaploid gametes (72.531-2 and 72.531-4) were hybridized to 83.242-l (designated as progenies 86.831 and 86.832). More than 200 of their hybrids were field evaluated near Davis in 1986. Open-pollinated seeds from the outcrossing female and self-fertile hermaphroditic decaploid Fig. 1. Fragaria virginiana x F. viridis interspecific hybrids. Top seedlings from these populations were germinated and grown left, pentaploid selection 83.242; top right, interspecific colchi-de- near Davis in 1987 and were designated progenies 87.813 and caploid 83.242-1; bottom left (magnification, × 146), the chromosal compliment of the pentaploid selection 83.242 (x = 7, 2n = 35); 87.814, respectively (Table 1, designated IIa-IIc and in Table bottom right (magnification, × 233), the chromosomal compliment 2). of the interspecific decaploid 83.242-l (x = 7, 2n = 70). III) 12x x 8x ® 10x or 16x x 4x ® 10x. The chromosome number of five octoploid cultivars was doubled and the derived fertile clones were hybridized to octoploid cultivars. All 12x colchicine (designated selection 83.242-1). The same procedure offspring were invariably sterile. It should be noted that the was followed to obtain a decaploid F. chiloensis (CMCl) × dodecaploid (12x, polytriploids) are fairly commonly found in ‘Alpine’ F. vesca hybrid (designated selection 85.926-l). Three octoploid breeding populations; they are absolutely sterile and previously doubled natural pentaploid interspecific hybrids of thus do not hybridize with octoploid cultivars (Table 1, desig- F. chiloensis and F. vesca from California were also incorpo- nated IIIa-IIId). rated into the current investigations. They were identified as A colchi-tetraploid clone of ‘Alpine’ F. vesca was synthe- female CA 1614 and hermaphrodites