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Chromosomal Polymorphism in Isolated Populations of Elymus (Agropyron) in the Aegean IV

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Chromosomal Polymorphism in Isolated Populations of Elymus (Agropyron) in the Aegean IV Hereditus 86: 237-244 (1977) Chromosomal polymorphism in isolated populations of Elymus (Agropyron) in the Aegean IV. General discussion WAHEEB K. HENEEN Institute of Genetics, 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 chromosomes. 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 chromosome 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 karyotype 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 heterochromatin 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 karyotypes 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.
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