Weeks S C, Crosser B R, Bennett R, Gray M, Zucker N
Department of Biology, University of Akron, Ohio 44325-3908, USA.
Evolution. 2000 Jun;54(3):878-87. doi: 10.1111/j.0014-3820.2000.tb00088.x.
Androdioecy is an uncommon form of reproduction in which males coexist with hermaphrodites. Androdioecy is thought to be difficult to evolve in species that regularly inbreed. The freshwater shrimp Eulimnadia texana has recently been described as both androdioecious and highly selfing and is thus anomalous. Inbreeding depression is one factor that may maintain males in these populations. Here we examine the extent of "late" inbreeding depression (after sexual maturity) in these clam shrimp using two tests: (1) comparing the fitness of shrimp varying in their levels of individual heterozygosity from two natural populations that differ in overall genetic diversity; and (2) specifically outcrossing and selfing shrimp from these same populations and comparing fitness of the resulting offspring. The effects of inbreeding differed within each population. In the more genetically diverse population, fecundity, size, and mortality were significantly reduced in inbred shrimp. In the less genetically diverse population, none of the fitness measures was significantly lowered in selfed shrimp. Combining estimates of early inbreeding depression from a previous study with current estimates of late inbreeding depression suggests that inbreeding depression is substantial (delta = 0.68) in the more diverse population and somewhat lower (delta = 0.50) in the less diverse population. However, given that males have higher mortality rates than hermaphrodites, neither estimate of inbreeding depression is large enough to account for the maintenance of males in either population by inbreeding depression alone. Thus, the stability of androdioecy in this system is likely only if hermaphrodites are unable to self-fertilize many of their own eggs when not mated to a male or if male mating success is generally high (or at least high when males are rare). Patterns of fitness responses in the two populations were consistent with the hypothesis that inbreeding depression is caused by partially recessive deleterious alleles, although a formal test of this hypothesis still needs to be conducted.
雄雌同体是一种罕见的繁殖形式，其中雄性与雌雄同体者共存。人们认为，在经常进行近亲繁殖的物种中，雄雌同体很难进化。德克萨斯真仙女虾这种淡水虾最近被描述为既是雄雌同体又是高度自交的，因此很反常。近亲繁殖衰退是可能使这些种群中雄性得以维持的一个因素。在此，我们使用两项测试来研究这些蚌虾中“晚期”（性成熟后）近亲繁殖衰退的程度：（1）比较来自两个总体遗传多样性不同的自然种群、个体杂合度水平各异的虾的适合度；（2）专门对来自这些相同种群的虾进行杂交和自交，并比较所得后代的适合度。近亲繁殖的影响在每个种群内部有所不同。在遗传多样性更高的种群中，近亲繁殖的虾的繁殖力、体型和死亡率显著降低。在遗传多样性较低的种群中，自交虾的任何一项适合度指标都没有显著降低。将先前一项研究中早期近亲繁殖衰退的估计值与当前晚期近亲繁殖衰退的估计值相结合表明，在多样性更高的种群中近亲繁殖衰退程度很大（δ = 0.68），而在多样性较低的种群中则略低（δ = 0.50）。然而，鉴于雄性的死亡率高于雌雄同体者，仅靠近亲繁殖衰退，这两个种群中近亲繁殖衰退的估计值都不足以解释雄性的维持。因此，只有当雌雄同体者在未与雄性交配时无法使许多自身卵子自受精，或者雄性的交配成功率普遍较高（或者至少在雄性稀少时较高），这个系统中雄雌同体的稳定性才有可能。两个种群中适合度反应的模式与近亲繁殖衰退是由部分隐性有害等位基因引起的这一假设一致，不过这一假设仍需进行正式检验。
