Som Christian, Bagheri Homayoun C, Reyer Heinz-Ulrich
Zoological Institute, University of Zürich, Zürich, Switzerland.
BMC Evol Biol. 2007 May 21;7:80. doi: 10.1186/1471-2148-7-80.
Female only unisexual vertebrates that reproduce by hybridogenesis show an unusual genetic composition. They are of hybrid origin but show no recombination between the genomes of their parental species. Instead, the paternal genome is discarded from the germline prior to meiosis, and gametes (eggs only) contain solely unrecombined maternal genomes. Hence hybridogens only transmit maternally inherited mutations. Hybridity is restored each generation by backcrossing with males of the sexual parental species whose genome was eliminated. In contrast, recombining sexual species propagate an intermixed pool of mutations derived from the maternal and paternal parts of the genome. If mutation rates are lower in female gametes than males, it raises the possibility for lower mutation accumulation in a hybridogenetic population, and consequently, higher population fitness than its sexual counterpart.
We show through Monte-Carlo simulations that at higher male to female mutation ratios, and sufficiently large population sizes, hybridogenetic populations can carry a lower mutation load than sexual species. This effect is more pronounced with synergistic forms of epistasis. Mutations accumulate faster on the sexual part of the genome, and with the purifying effects of epistasis, it makes it more difficult for mutations to be transmitted on the clonal part of the genome. In smaller populations, the same mechanism reduces the speed of Muller's Ratchet and the number of fixed mutations compared to similar asexual species.
Since mutation accumulation can be less pronounced in hybridogenetic populations, the question arises why hybridogenetic organisms are so scarce compared to sexual species. In considering this, it is likely that comparison of population fitnesses is not sufficient. Despite competition with the sexual parental species, hybrid populations are dependent on the maintenance of--and contact with--their sexual counterpart. Other problems may involve too little genetic diversity to respond to changing environments and problems in becoming hybridogenetic (e.g. disruption of meiosis and subsequent infertility or sterility). Yet, lower mutation accumulation in hybridogenetic populations opens the possibility that hybridogenetic species can develop into new sexual species once recombination is re-established and reproductive isolation from sexual ancestors has occurred.
通过杂交起源进行繁殖的单性脊椎动物表现出独特的基因组成。它们起源于杂交，但亲本物种的基因组之间不发生重组。相反，父本基因组在减数分裂前从生殖系中被丢弃，配子（仅卵子）仅包含未重组的母本基因组。因此，杂交起源物种只传递母系遗传的突变。通过与基因组被消除的有性亲本物种的雄性回交，杂交性在每一代得以恢复。相比之下，进行重组的有性物种传播的是源自基因组母本和父本部分的混合突变库。如果雌配子中的突变率低于雄配子，那么杂交起源种群中突变积累减少的可能性就会增加，进而其种群适应性可能高于有性对应物种。
我们通过蒙特卡洛模拟表明，在雄雌突变率比值较高且种群规模足够大时，杂交起源种群的突变负荷可能低于有性物种。这种效应在协同上位性形式下更为明显。突变在基因组的有性部分积累得更快，并且由于上位性的纯化作用，使得突变在基因组的克隆部分传递变得更加困难。在较小种群中，与类似的无性物种相比，相同机制降低了穆勒棘轮的速度和固定突变的数量。
由于杂交起源种群中的突变积累可能不太明显，于是产生了一个问题，即为什么与有性物种相比，杂交起源生物如此稀少。在考虑这个问题时，仅比较种群适应性可能并不充分。尽管与有性亲本物种存在竞争，但杂交种群依赖于有性对应物种的维持以及与其接触。其他问题可能包括遗传多样性过少而无法应对不断变化的环境，以及形成杂交起源的过程中出现的问题（例如减数分裂中断以及随后的不育或不孕）。然而，杂交起源种群中较低的突变积累开启了一种可能性，即一旦重新建立重组并与有性祖先实现生殖隔离，杂交起源物种可能会发展成为新的有性物种。
