Does reproductive output differ between related and unrelated male-female pairs of guppies, Poecilia reticulata?

Mating with close relatives (‘inbreeding’) is common in small, fragmented populations. Inbreeding leads to a higher frequency of loci with homozygous recessive alleles, which can have serious consequences for offspring fitness (‘inbreeding depression’). In addition, females may differentially invest resources when they mate with a related or a nonrelated male, which might affect offspring fitness. A decline in the value of traits of inbred offspring, particularly traits displayed early in life, may therefore be caused by lower maternal investment when females mate with a relative (i.e. differential allocation) rather than solely being attributable to greater homozygosity of inbred offspring. In this study, we mated female guppies (<span type="Italic" name="Emphasis" class="Italic">Poecilia reticulata</span>) to a brother or an unrelated male. We then measured the proportion of females breeding, their gestation time, offspring number, and offspring size at birth. We also tested if offspring traits are related to their mother’s size, and their father’s sexual coloration (‘attractiveness’). Mating with a brother did not lower the gestation time, or the number or the size of offspring at birth. However, smaller females gave birth to fewer, smaller babies; and females mated to males with more black coloration gave birth significantly sooner. In addition, females were more likely to give birth when mated to a male with more black colouration, but only when he was an unrelated male, rather than their brother. In sum, reproductive success did not differ when a female mated with a brother or unrelated male. There was no evidence for either inbreeding depression or differential maternal allocation on early life history traits when mating with a relative.

近亲繁殖（又称‘近交’）在小型、破碎化的种群中较为常见。近交现象导致同一位点隐性等位基因纯合频率升高，进而可能对后代适应性（‘近交衰退’）产生严重影响。此外，雌性在与其近亲或非近亲雄性交配时，可能会差异性地分配资源，这可能会影响后代的适应性。因此，近交后代性状价值的下降，尤其是早期生命阶段展现的性状，可能并非仅仅归因于近交后代更高的纯合度，而是由于雌性在与其近亲交配时母性投资减少（即差异分配）所致。在本研究中，我们将雌性孔雀鱼（Poecilia reticulata）与兄弟或非近亲雄性进行交配。随后，我们测量了繁殖雌性的比例、妊娠时间、后代数量以及出生时后代的体型。我们还测试了后代性状与其母亲体型和父亲性色（‘吸引力’）之间的关系。与兄弟交配并未降低妊娠时间，或出生时后代的数量和体型。然而，体型较小的雌性生育的后代数量较少且体型较小；与性色中黑色成分更多的雄性交配的雌性，其分娩时间显著提前。此外，雌性在与其黑色性色更丰富的雄性交配时更有可能分娩，但这种倾向仅在雄性为非近亲而非兄弟时出现。总之，雌性与兄弟或非近亲雄性交配时，其繁殖成功率并无差异。在与其近亲交配时，没有证据表明存在近交衰退或母性在早期生命历史性状上的差异分配。