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 (Poecilia reticulata) 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.