Hereditus 86: 237-244 (1977) Chromosomal polymorphism in isolated populations of Elymus (Agropyron) in the Aegean IV. General discussion

WAHEEB K. HENEEN Institute of , University of Lund, Sweden

HENEEN,W. K. 1977. Chromosomal polymorphism in isolated populations of EIymus (Agropyron) in the Aegean. IV. General discussion. - Hereditas 86: 237-244. Lund, Sweden. ISSN 0018-0661. Received April 27, 1977 A striking chromosomal polymorphism predominates in Elymus striatulus (2n = 14), E. rechingeri (2n = 28) and E. diue (2n = 56) which grow in small isolated populations largely on the Aegean islands. The polymorphism was expressed mainly in the existence of several variant types of the satellite . Satellite chromosomes might be more susceptible to structural alterations due to the presence of secondary constrictions, nucleolar organizers, heterochromatic blocks and repetitive DNA which are known to be labile sites. Chromosomal polymorphism was more pronounced in the two polyploid species which might be attributed to the compensatory effect of the high number of homologous, homoeologous or related genomes in the polyploids. Chromosomal polymorphism was also favored by the rhizomatous or tufted patterns of vegetative growth of these species. The evolutionary aspects of the chromosomal, genetical and morphological differences between populations might be related to (I) inbreeding of crossfertilizers and selection for chromosomal heterogeneity and/or (2) drift or a founder effect which is also supported by the paleogeographical records. The species in the Aegean area are apparently in an active phase of evolutionary change. W. K. Heneen, Institute of Genetics, University of Lund, S-223 62 Lund. Sweden

Elymus striatulus (2n = 14), E. rechingeri (2n = 28) and to at least one genome. Variants of satellite types A E. dine (2n=56) are three littoral species found by and B as well as variants of type C (found only in RUNEMARKin the Aegean. They grow in small isolated E. rechingeri) were present either in a homo- or populations. Seeds collected from natural populations heterozygous condition. mainly on the Aegean islands were grown under Satellite constitutions that were most greenhouse conditions. Cytological analysis of the common and thus most probably characteristic for plants raised from these seeds provided evidence for the species were A2A2 BIBl for E. striatulus, A2A2 the prevalence of chromosomal polymorphism in the BIB1C2C2 for E. rechingeri and possibly AA AA BB three species (HENEENand RUNEMARK1962, 1969, for E. diue. The original papers (cited above) should 1972; HENEEN1977a, b, c). This polymorphism be consulted for details on numerical, structural and was manifested in the variable appearance of the pairing peculiarities of the chromosomes in the three satellite chromosomes and the presence of structurally species and in the offspring from inter- and intra- new chromosome types (markers). population crosses in the case of E. rechingeri. In Table 1 a summary is presented of the data on The present paper is a general discussion of the numbers of plants studied and the populations chromosomal polymorphism observed in the Aegean they represented, and also the number of satellite Elymus material. and marker chromosome constitutions present. The appearance of the different types of the satellite and 1. Nature of chromosomal polymorphism marker chromosomes is shown schematically in Fig. 1. Satellite chromosome types A and B were The chromosomal polymorphism in the Aegean found in all three species which indicates a similarity Elymus material was expressed as diversity in the or close similarity between these species with respect appearance of the satellite chromosomes and the 238 W. K. HENEEN Hereditus 86 (1977)

Table 1. Number of plants, populations, and satellite expressed in altered banding patterns is much more and marker chromosome constitutions encompassed frequent than has been anticipated, when considering in the EIymus material studied the concept of constancy. The chromo- somal polymorphism in the Elymus material was No. of No. of No. of readily detectable in the Feulgen-stained (OSTERGREN plants popula- chromo- and HENEEN1962) chromosomes, being manifested as tions some con- conspicuous alterations in these chromosomes. Thus, stitutions the structural variations here are on a gross level if compared with polymorphic banding patterns. E. striatulus (2n = 14) 26 9 6 E. rechingeri (2n = 28) 45 20 21 The high variability in the appearance of the E. diue (2n = 56) 20 4 6 satellite chromosomes might be correlated with the specific structural organization of these chromo- somes. Regions of secondary constrictions in the presence of marker chromosomes. There were many Elymus material are most probably sites of nucleolar variants of the satellite chromosomes and a few organisation. It is also common that the presence of marker types. Marker chromosomes were found only secondary constrictions is connected with a hetero- in the tetraploid and octoploid species. Apparently, chromatic constitution. The application of banding the satellite chromosomes were more susceptible to techniques on plant materials disclosed the presence structural alterations than the other chromosomes. of at secondary constrictions and Banding techniques and detailed karyotype analysis nucleolar sites (e.g. VOSA 1973; 1976b; MARKSand would be required for a more exact estimation of SCHWEIZER1974; FISKESJ~1975; WEIMARCK1975; structural alterations in the different chromosome SCHWEIZERand EHRENDORFER1976). It is also well types in the complement. documented that repetitive sequences of DNA occur Of interest here is the occurrence of what might be at nucleolar organizer sites (BIRNSTIELet al. 1966; the same variant chromosome type in more than one PARDUE1975) and in the heterochromatic blocks species (Fig. 1). Thus, types A6, B3 and B4 were in (e.g. HOLMQUIST1975; TIMMEet al. 1975; NARAYAN common for E. striatulus and E. rechingeri and types and REES1976). That structural rearrangements occur D(A,) and MI for E. rechingeri and E. diae. That more frequently in heterochromatin is well established these chromosome types, which are morphologically (see JOHNand LEWIS1968). Also more breaks are similar at the present resolution, really correspond to usually encountered at sites of secondary constric- each other needs further verification by banding tions. Secondary constrictions are potentially ‘weak’ techniques. If this turns out to be true, it would points where breakage due to physical strain is likely provide evidence that genome similarities exist to occur. The lability of repetitive DNA has also been between these species and that corresponding chro- documented (BRITTENand KOHNE 1968; FLAMM mosomes might undergo similar structural alterations 1972; FLAVELL~~al. 1974; HOLMQUIST1975; NARAYAN in the different species. Another possibility would be and REES 1976). that chromosome similarities between species is an Evidently secondary constrictions, nucleolar or- indication of distant hybridisation between ancestral ganizers, heterochromatic blocks and repetitive DNA taxa. A third alternative would be that these variant are all liable sites and are more susceptible to types were already there in a common ancestor changes than other sites. A relatively high susceptibil- before the differentiation of the present taxa. More ity would result when a combination or all of these than one mode of origin could be valid in the present characters concur. material. The probable presence in the three species Structurally deviating satellite and nucleolar chro- of common genomes might be of relevance in con- mosomes have been described in a variety of nection with the prevailing chromosomal poly- materials. In the present context, reference will be morphism. made to some examples of polymorphisms in the The use of banding techniques has proved useful appearance of these chromosomes in natural plant for interspecific comparisons of and populations (JONES1964; MARCHANTand BRIGHTON satellite chromosomes (e.g. GILLand KIMBER1974; 1971; FUJISHIMAand KURITA 1973; VOSA 1973, MARKSand SCHWEIZER1974; FISKEBJ~1975; GREIL- 1976b; BOUG~URDand PARKER1976). In the Aegean, HUBER and SPETA1976; SCHWElZER and EHRENDORFER chromosomal polymorphism was predominant in 1976; VOSA 1976a). By the application of banding satellite chromosomes of Nigella, Leopoldia, AIIium techniques in plant, animal and human materials, it and Fritillaria (STRID 1965, 1969; BENTZER1969, is now well established that structural polymorphism 1972a and b, 1974; BOTHMER1970,1975; ENGSTRAND Hereditas 86 (1977) CHROMOSOMAL POLYMORPHISM IN ELYMUS IV 239 240 W. K. HENEEN Hereditas 86 (1977) and GUSTAFSSONin RUNEMARK1970) also in non- from nature indicates the survival and preservation satellite chromosomes (STRID1968; BENTZER1972b; of many of these structural alterations. BOTHMER1970). Thus cytological instability is a The rhizomatous and tufted patterns of vegetative common phenomenon for a variety of Aegean species growth in the present material are thus highly that belong to different orders. advantageous since they allow for more tolerance to Evidently, the various types of satellite chromo- chromosome structural alterations and low fertility. somes found in the Elymus material are heritable The frequent occurrence of structural alterations in entities. This was indicated especially from crosses the chromosomes of vegetatively reproducing plants involving E. rechingeri. In the progeny of these has been pointed out in a variety of materials (e.g. crosses, expected constellations of satellite chromo- LEVAN1935; DYER1963; VOSA1973). somes were obtained. The preservation of satellited types has also been shown in crosses made in other species showing chromosomal polymorphism (STRID 4. Evolutionary aspects in a small isolated population 1965, 1969; BOUGOURDand PARKER1976). system A. Inbreeding of cross-fertilizers and selection for 2. level and the extent of chromosomal structural diversity polymorphism The Aegean Elymus species studied are cross-fertil- The material analyzed of the three Aegean Elymus izers, nevertheless capable of self-fertilization. Struc- species, although limited, gave an indication of a tural heterozygosity is a common phenomenon in more extensive chromosomal polymorphism in the cross-fertilizers (MUNTZING1945). Thus, a certain polyploid species E. rechingeri and E. diae than in the degree of the chromosomal polymorphism observed diploid E. striatulus (Table 1). There is probably a in the Elymus material might be attributable to such a higher tolerance of structural rearrangements in the structural heterozygosity. polyploid species. The presence of higher numbers of In nature, the small size of the populations creates genomes in the polyploids and a possible partial inadequate conditions for cross-fertilization. It is homology or homoeology between these genomes very likely that this would lead to inbreeding. This is would buffer possible deleterious effects associated supported by the observed low fertility both in with structural alterations. nature and in cultivation. Thus the different popula- The present findings are in agreement with the tions might represent a variety of inbred lines. reported higher frequencies of chromosome structural Structural differences have been documented in in- and numerical changes in polyploids (e.g. BENTZER bred lines of cross-fertilizers (e.g. HENEEN1962). 1969; BRANDHAM1974). The observed structural polymorphism within Elymus populations, especially in the case of E. rechingeri, might be causally connected with inbreed- 3. Vegetative propagation and chromosomal ing. It has been pointed out that inbreeding of polymorphism outbreeding species favors the preservation of genetic Vegetative propagation is an advantageous means to variability (see ALLARDet al. 1968). Chromosomal continued survival and reproduction in plants that polymorphism is possibly maintained in the Aegean are sterile or have diminished fertility as a conse- by means of adaptive mechanisms. This could be a quence of changes in the chromosome structure or the consequence of heterozygous advantage or selection genetic make-up. All three Elymus species are for diversity in a situation of forced inbreeding perennials, E. striatulus vegetatively propagating (DARLINGTON1963; JOHNand HEWITT 1966; SNOGE- by rhizomes, whereas E. rechingeri and E. diae are RUP 1967; ALLARDet al. 1968; METTLERand GREGG tufted grasses. These aspects are evidently of relevance 1969). in the maintenance and survival of plants with It would be of interest to further elucidate the chromosome structural rearrangements which other- extent of chromosomal polymorphism within versus wise might be deleterious (HENEEN1963). between populations in the Elymus material. The In natural Elymus populations, seed setting was material studied is too limited to clarify this point found to be low. The low fertility was especially adequately. From the information available, it ap- pronounced in E. rechingeri. In certain populations pears that variability within populations was most of this species, no or only few seeds were found. The common in E. rechingeri. In this species, chromosomal finding of a wide spectrum of satellite chromosome polymorphism was observed among plants origi- constitutions in the plants raised from seeds collected nating from 9 out of 11 populations from which at Herediras 86 (1977) CHROMOSOMAL POLYMORPHISM IN ELYMUS IV 241

least two individuals were analyzed. The corre- sponding figures for E. striatulus and E. diae were 3 C. Paleogeography out of 8 and 1 out of 4, respectively. Chromosomal The geological history of the Aegean area (CREUTZ- polymorphism within populations of other Aegean BURG 1963, 1966) has been discussed and commented species has also been pointed out by STRID(1968), upon by several workers (SNOGERUP1967; STRID BENTZER(1969) and BOTHMER(1 975). 1970, 1971; RUNEMARK1970, 1971a, b). In short, there was a land bridge between southern Greece and western Turkey during the Pliocene. The land B. Drgt and founder effect bridge was bordered northward by a large lake In a small isolated population system, there is an covering most parts of the North Aegean Sea, increased chance of random fixation of alleles and and southward by the Sea of Crete separating chromosome structural changes (WRIGHT1931,1940, it from the south Aegean islands. At the beginning 1941). A SEWALLWRIGHT effect is very likely to be of of the Pleistocene, the land bridge was broken great evolutionary significance for various plant down into the present central Aegean archipelago. species in the Aegean area (RUNEMARK1970). The The islands were then subjected to climatic fluctua- possibility of random fixation of structural and genet- tions, pluvial periods, tectonic activities and sea ical peculiarities has also been pointed out in other regressions and transgressions. The bearing upon Aegean species (SNOGERUP1967; STRID1970; BENT- of these aspects on phenomena such as forced ZER 1973; BOTHMER1974) as well as in other materials inbreeding, drift and founder effect in a small growing in small populations (e.g. JAINand RAI1974). isolated population system is apparent. The central The small populations on the Aegean islands may and southern Aegean islands most likely have attribute their origin to a founder effect (MAYR1963). served as refugia for the last remnants of the Pliocene This is not in disagreement with the paleogeographical flora around the Sea of Crete (RUNEMARK1971b). history of this area as outlined below. The conditions of the studied small populations Whether the observed polymorphism in the Aegean on the Aegean islands are comparable to the situation is a consequence of drift or founder effect, one would on mountain ranges where heterogeneity of chromo- expect to find interpopulation differences not only in some composition also predominates (FUKUDAand chromosomal patterns but also in genetical and CHANNELL1975). This heterogeneity was attributed morphological criteria. to the variable and fluctuating climatic and paleo- Data from crossing experiments were obtained for geologic habitat conditions. E. rechingeri. There was a higher seed set in intra- population crosses than in inter-population crosses. This was attributed to differences in the genetical 5. Concluding remarks make-up of these small isolated populations. The Chromosomal polymorphism prevails not only in lower fertility in the case of inter-population crosses EIymus species but also in several other Aegean cannot be due solely to chromosomal polymorphism species. In many of these instances, also morphologi- since this was also predominant within populations. cal diversity was predominant between populations. Of the three EIymus species studied, E. rechingeri Within populations, however, the plants were very and to some extent E. diae were morphologically uniform. Thus, it appears that the factors operating highly polymorphic. Great dissimilarities prevailed in the Aegean have similar effects on many species between populations, but within populations, the that belong to a wide spectrum of families and which plants were morphologically uniform. This was also represent annuals and perennials, diploids and poly- the case in several other species on the Aegean islands ploids, also self-fertilizers and cross-fertilizers. ( RUNEMARK197 1a). Since chromosomal polymor- From an evolutionary point of view, it would phism was common both between and within Elymus appear that the following factors are most relevant populations, it was inferred that there is no correla- in connection with the predominating chromosomal tion between morphological and cytological diver- polymorphism in these materials: (1) High mutability sities between populations. A similar situation has rates associated with an adaptive preference and been described in other materials such as Glandulariu natural selection of structural heterogeneity in cross- (SOLBRIG1968) and Nigella (STRID1970). fertilizers that are highly inbred. (2) Drift and The observed morphological uniformity within random fixation of structural and genetical devia- populations and diversity between populations is tions in a small isolated population system. probably a consequence of genetic drift and fixations The rate of evolution in small populations is rather of genes in a small isolated population system. rapid. At a later phase the present state might lead 242 w. K. HENEEN Hereditas 86 (1977) to effective competitive selection between population DARLINGTON,C. D. 1963. Chromosome Botany. - George groups and to reproductive drift (RUNEMARK1969) Allen & Unwin, London DYER,A. F. 1963. Allocyclic segments of chromosomes and in less adapted populations. 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