ヤマトシロアリの配偶システムと条件的単為生殖

Both Social Hymenoptera (ants, bees and wasps) and Isoptera (termites) have independently evolved eusociality based on similar colony structures. In social Hymenoptera, queens found their colonies either solitarity or together with other newly mated queens. Males never contribute to colony foundation and social labor. In contrast to social hymenopterans, termite colony foundation usully involves both sexes. However, pairing efficiency tends to be low in the termite Reticulitermes speratus because of a limited mate-searching range, the female-biased sex ratio and a relatively low calling ability. Recently, facultative parthenogenesis was revealed to have important adaptive significance in R. speratus. Female alates that fail to pair with males found colonies cooperatively with partner females or even alone and reproduce by parthenogenesis in this species. Even female-female coopertion promotes colony foundation success if pairing with males is not possible. Facultative parthenogenesis is advantageous, even if ultimayely it may be inferior to sexual reproduction in terms of long term fitness. It enables female to reproduce in the event that a mating partner cannot be found. Diploid female progency that reproduced by thelytokous parthenogenesis showed normal development. Chromosome observation and genetic analysis using microsatellites showed that the restoration of ploidy is accomplished terminal fusion. In this paper, I discuss asexual colony foundation, mechanism of terminate parthenogenesis, selection factors for the evolution of parthenogenesis and the costs of asexual reproduction.

ヤマトシロアリの配偶システムと単為生殖に関する最新の研究結果をレビューした．まず，シロアリの生活史について述べ，特に群飛から異性探索，そしてコロニー創設に至るまでの行動特性に関する新たな知見について述べた．ヤマトシロアリの雌の有翅虫は，群飛後に雄と配偶できなかった場合，単独，または二雌の共同でコロニーを創設し，単為生殖により繁殖することが明らかになった．単為生殖で産まれた子は，わずかな卵期間の延長を除けば，有性生殖の子と同様に正常発育する．染色体観察およびマイクロサテライト遺伝子解析により，ヤマトシロアリの単為生殖では末端融合型のオートミクシスにより二倍体に核相回復することが示された．さらに，単為生殖の進化に必要な条件，単為生殖の伴う遺伝的・発生的制約に加え，生態学的制約について考察した．