Evolutionary patterns of mating systems in the genus Closterium inferred from molecular information.

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The genus Closterium contains more than 70 species each comprising varieties. Some sexual populations form zygospores within single clones (homothallism), while some others between clones (heterothallism). There are also asexual populations which form thick-walled resistant spores without conjugation (parthenogenesis). In order to get better understanding of speciation of the genus along with evolutionary patterns of their reproductive types, this study has primarily treated with three closely-related species complex : C. moniliferum-ehrenbergii ; C. calosporum and ; C peracerosum-strigosum-littrale (psl). Phylogenetic trees inferred from both nuclear SSU rDNA and chloroplast-encoded rbcL generally supported diversification into three different clades in each of those species complex. In the C moniliferum-ehrenbergii species complex, the phylogenetic tree built from SSU rDNA data has suggested that homothallic C moniliferum v. moniliferum was ancestral to all other populations of this species complex. ML trees constructed from SSU 1506 group I intron and ITS-2 data has shown that populations of C ehrenbergii were not monophyletic. A characteristic duplication at the tip of helix I of their ITS-2 secondary structure may serve as an important clue for evolutionary derivation of asexual populations from sexual ones. In the C. calosporum complex, each of the two species C. calosporum and C. spinosporum has turned out to be polyphyletic. On helix II of their ITS-2 secondary structure of this species complex, a characteristic motif found in homothallic C. calosporum v. himalayense was duplicated in heterothallic C. calosporum v. galiciense. Since these two taxa are most closely related with each other, one explanation for this may be that heterothallic galiciense has been derived from a homothallic himalayense-like ancestor. In the C. psl species complex, unfortunately, neither sequence nor structure characteristic for their each reproductive type was found so far.