No evidence of inbreeding depression in sperm performance traits in wild song sparrows

Abstract

Inbreeding is widely hypothesized to shape mating systems and population persistence, but such effects will depend on which traits show inbreeding depression. Population and evolutionary consequences could be substantial if inbreeding decreases sperm performance and hence decreases male fertilization success and female fertility. However, the magnitude of inbreeding depression in sperm performance traits has rarely been estimated in wild populations experiencing natural variation in inbreeding. Further, the hypothesis that inbreeding could increase within‐ejaculate variation in sperm traits and thereby further affect male fertilization success has not been explicitly tested. We used a wild pedigreed song sparrow (Melospiza melodia) population, where frequent extrapair copulations likely create strong postcopulatory competition for fertilization success, to quantify effects of male coefficient of inbreeding (f) on key sperm performance traits. We found no evidence of inbreeding depression in sperm motility, longevity, or velocity, and the within‐ejaculate variance in sperm velocity did not increase with male f. Contrary to inferences from highly inbred captive and experimental populations, our results imply that moderate inbreeding will not necessarily constrain sperm performance in wild populations. Consequently, the widely observed individual‐level and population‐level inbreeding depression in male and female fitness may not stem from reduced sperm performance in inbred males.

Low hatching success in the critically endangered kākāpō is driven by early embryo mortality not infertility

“…Moreover, early embryo death is more likely to be related to heterozygosity of the embryo itself than that of either parent and is an expected consequence of inbreeding as inbred individuals express more lethal recessives during development (Charlesworth & Willis, 2009). In contrast, the evidence that inbreeding depresses adult sperm function and individual fertility in wild populations is equivocal (Losdat, Chang & Reid, 2014; Losdat et al ., 2018). It therefore seems likely that early embryo mortality in kākāpō is largely driven by inbreeding depression on early survival.…”

Relationship between semen quality and level of heterozygosity on the model of endangered population of Western capercaillie

“…Furthermore, inbreeding negatively influenced sperm quality in Mexican grey wolves (Asa et al, 2007) and white‐footed mouse (Malo et al, 2010). On the other hand, lack of relationship between inbreeding and sperm traits was showed for song sparrows (Losdat et al, 2018), whooping cranes (Brown et al, 2015), rabbits (Gage et al, 2006), Mohor gazelle and Iberian lynx (Ruiz‐López et al, 2012). Thus, we can say that relationship between heterozygosity and sperm quality is in many cases species specific.…”

Low hatching success in the critically endangered kākāpō (Strigops habroptilus) is driven by early embryo mortality not infertility

“…Moreover, early embryo death is more likely to be related to heterozygosity of the embryo itself than that of either parent, and is an expected consequence of inbreeding as inbred individuals express more lethal recessives during development (Charlesworth & Willis 2009). In contrast, the evidence that inbreeding depresses adult sperm function and individual fertility in wild populations is equivocal (Losdat et al 2014(Losdat et al , 2018. It therefore seems likely that early embryo mortality in kākāpō is largely driven by inbreeding depression on early survival.…”