Abstract
As populations decline, inbreeding becomes increasingly unavoidable. Increased genome-wide homozygosity for inbred individuals can result in reduced survival and reproductive fitness (i.e. inbreeding depression), via the expression of deleterious recessive alleles and reduced heterozygosity at over-dominant loci. The tendency for deleterious recessive alleles to drift to high frequencies in small populations means that inbreeding depression can be particularly severe for threatened populations. Therefore, the genetic consequences of small and isolated populations are becoming of increasing concern to conservation biologists.
The kakapo (Strigops habroptilus) is a critically endangered, flightless, nocturnal parrot that now survives only on predator-free island sanctuaries in New Zealand. The recent population bottleneck of 51 individuals, lek mating system and insular origin of all but one of the surviving kakapo, render them particularly susceptible to inbreeding depression. Low productivity of kakapo has been reported and potentially attributed to, an aging population, diet and inbreeding. For this reason, the present study investigated the relationship between inbreeding and reproductive fitness in kakapo.
The preferred method of assessing inbreeding is to use multi-generational pedigree information. However, this is currently unavailable for kakapo, therefore molecular estimates of relatedness were used as a surrogate for pedigree-derived inbreeding coefficients. Internal relatedness and pairwise relatedness were calculated using 25 polymorphic microsatellite loci. The link between relatedness estimates and variation in early life history traits was investigated using heterozygosity-fitness correlations. This was achieved through the use of generalised linear mixed modelling, with an information-theoretic approach and model averaging where necessary. The reproductive traits investigated were female fecundity (clutch size), egg fertility (probability of an egg being fertilised by an individual male), hatching success of fertile eggs (proportion of fertile eggs that a female hatches) and sperm quality (concentration, motility and morphology).
Neither variation in female fecundity nor egg fertility could be attributed to homozygosity with any confidence. Hatching success was determined to be strongly reduced for the more homozygous females and the more homozygous males were determined to have significantly higher proportions of abnormal sperm. Therefore, the present study demonstrated that inbreeding depression is a contributing factor towards reduced reproductive success in kakapo. These findings are consistent with known detrimental effects of inbreeding, in particular for New Zealand endemic species that have experienced severe population bottlenecks. To minimise these effects, and hence reduce potential impacts on population growth and species recovery, kakapo managers should aim to prevent further erosion of genetic diversity and to breed from the descendants of the genetically-distinct Fiordland male Richard Henry. The present study highlights the importance of considering the genetic components of populations in any conservation management program.