Differentiation of the StY genomes in Elymus species as referred by meiotic pairing in interspecific hybrids and its evolutionary significance
LU Bao-Rong, Bjφrn Salomon
Biodiv Sci. 2004, 12 (2):
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Hybridization-differentiation is a major pathway of speciation and the active force of plant evolution, including homoploid speciation without change of chromosome numbers. Effective isolation between parental species and their hybrid derivatives plays an essential role in homoploid speciation for stabilizing a newly formed species. Studies of isolation mechanisms will facilitate our understanding of the speciation process. The genus Elymus of Triticeae (Poaceae) includes ca. 150 polyploid species with different genomes, viz. the StH, StY, StHY, StPY, and StWY from different origins, providing an ideal group for studying mechanisms of polyploid and homoploid speciation. There are about 30 tetraploid species containing the StY genomes distributed in temperate Asia and the eastern margin of Europe. In order to study genomic relationships of the StY Elymus species, samples representing 26 species collected from Western through Eastern Asia were extensively hybridized with each other. Meiotic pairing at metaphaseI of the intra- and interspecific hybrids was analyzed, which revealed a significant differentiation pattern in homology of the StY genomes among Elymus species studied. Species from the same regions, e.g. within eastern or western Asia, shared the StY genomes with a relatively low level of differentiation, but species from different regions, e.g. between eastern and western Asia, shared substantially differentiated StY genomes. Species from Central Asia contained intermediately differentiated StY genomes compared with those from western and eastern Asia. The discovery of geographical differentiation of the StY genomes in tetraploid Elymus species has significance for study of evolutionary processes and mechanisms of homoploid speciation in Elymus. In addition to other wellrecognized factors responsible for the isolation between parental species and their hybrid derivatives during the hybridizationdifferentiation process (such as temporal, spatial, genic, and ecological isolation), the authors believe that meiotic irregularity caused by genomic differentiation between species also provides an important mechanism for homoploid speciation.