Hebert Paul D N, Beaton Margaret J, Schwartz Steven S, Stanton David J
Department of Biological Sciences, University of Windsor, Windsor, ON, N9B 3P4, CANADA.
Evolution. 1989 Aug;43(5):1004-1015. doi: 10.1111/j.1558-5646.1989.tb02546.x.
There is growing evidence that transitions from sexual to asexual reproduction are often provoked by internal genetic factors rather than extrinsic selection pressures. In the cladoceran crustacean Daphnia pulex, the shift to asexuality has been linked to sex-limited meiosis suppression. Most populations of this species reproduce by obligate parthenogenesis, but cyclically parthenogenetic populations persist in the southern portion of its range. The meiosis-suppressor model predicts that asexuality in D. pulex has polyphyletic origins and that the coexistence of cyclically parthenogenetic lines with male-producing obligately asexual clones should be unstable. For the present study, we examined the genotypic structure of D. pulex populations from a region in which there is an abrupt microgeographical shift in breeding system. Populations in Michigan largely reproduce by cyclic parthenogenesis, while those in Ontario are obligately asexual. Allozyme studies on 77 populations from this area revealed 50 obligately asexual clones, divisible into two groups: one derived from a single parent species and the other derived via interspecific hybridization. Although nearly 50% of the clones retained male production, there was, as predicted, no evidence of coexistence between cyclically parthenogenetic populations and male-producing obligately asexual clones. The survey did, however, reveal a low incidence of cyclically parthenogenetic populations in Ontario. The high genotypic diversity of these populations suggests that they are not only resistant to meiosis suppression, but able to rework genetic variation gained from asexual clones into a sexual breeding system.
越来越多的证据表明，从有性生殖向无性生殖的转变通常是由内部遗传因素而非外部选择压力引发的。在枝角类甲壳动物蚤状溞中，向无性生殖的转变与性别受限的减数分裂抑制有关。该物种的大多数种群通过专性孤雌生殖进行繁殖，但周期性孤雌生殖种群在其分布范围的南部持续存在。减数分裂抑制模型预测，蚤状溞的无性生殖具有多系起源，并且周期性孤雌生殖品系与产生雄性的专性无性克隆的共存应该是不稳定的。在本研究中，我们检查了来自一个繁殖系统存在突然微地理转变的地区的蚤状溞种群的基因型结构。密歇根州的种群主要通过周期性孤雌生殖进行繁殖，而安大略省的种群则是专性无性生殖。对该地区77个种群的等位酶研究揭示了50个专性无性克隆，可分为两组：一组来自单一亲本物种，另一组通过种间杂交产生。尽管近50%的克隆保留了雄性生产能力，但正如预测的那样，没有证据表明周期性孤雌生殖种群与产生雄性的专性无性克隆之间存在共存。然而，该调查确实揭示了安大略省周期性孤雌生殖种群的发生率较低。这些种群的高基因型多样性表明，它们不仅对减数分裂抑制具有抗性，而且能够将从无性克隆获得的遗传变异重新纳入有性繁殖系统。
