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Abstract The degree to which individuals inbreed is a fundamental aspect of population biology shaped by both passive and active processes. Yet, the relative influences of random and non‐random mating on the overall magnitude of inbreeding are not well characterized for many taxa. We quantified variation in inbreeding among qualitatively accessible and isolated populations of a sessile marine invertebrate (the colonial ascidianLissoclinum verrilli) in which hermaphroditic colonies cast sperm into the water column for subsequent uptake and internal fertilization. We compared estimates of inbreeding to simulations predicting random mating within sites to evaluate if levels of inbreeding were (1) less than expected because of active attempts to limit inbreeding, (2) as predicted by genetic subdivision and passive inbreeding tolerance, or (3) greater than simulations due to active attempts to promote inbreeding via self‐fertilization or a preference for related mates. We found evidence of restricted gene flow and significant differences in the genetic diversity ofL. verrillicolonies among sites, indicating that on average colonies were weakly related in accessible locations, but their levels of relatedness matched that of first cousins or half‐siblings on isolated substrates. Irrespective of population size, progeny arrays revealed variation in the magnitude of inbreeding across sites that tracked with the mean relatedness of conspecifics. Biparental reproduction was confirmed in most offspring (86%) and estimates of total inbreeding largely overlapped with simulations of random mating, suggesting that interpopulation variation in mother–offspring resemblance was primarily due to genetic subdivision and passive tolerance of related mates. Our results highlight the influence of demographic isolation on the genetic composition of populations, and support theory predicting that tolerance of biparental inbreeding, even when mates are closely related, may be favoured under a broad set of ecological and evolutionary conditions.
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This content will become publicly available on November 1, 2025
Inbreeding avoidance and cost in a small, isolated trout population
The persistence of small populations is influenced by the degree and cost of inbreeding, with the degree of inbreeding depending on whether close-kin mating is passively or actively avoided. Few studies have simultaneously studied these factors. We examined inbreeding in a small, isolated population of westslope cutthroat trout using extensive genetic and demographic data. Passive inbreeding avoidance was low, with predicted lifetime dispersal of approximately 36 and 74 m for females and males, respectively. Additionally, we found limited evidence for active inbreeding avoidance during reproduction. Relatives remained spatially clustered into adulthood, and observed relatedness among mate pairs was greater than expected under random mating by 0.09, suggesting that inbreeding is a concern in this population. Further, we examined sex-specific inbreeding depression throughout the life cycle and provide evidence for inbreeding depression in some fitness components, including family size, juvenile survival and reproductive success. Our results suggest that, in an at-risk trout population, limited passive and active inbreeding avoidance lead to a higher degree of inbreeding than expected under random mating. Observed inbreeding, along with evidence for fitness reduction due to inbreeding depression, could put the population at a heightened risk of decline or extirpation.
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- Award ID(s):
- 1652278
- PAR ID:
- 10613571
- Publisher / Repository:
- Proceedings of the Royal Society B
- Date Published:
- Journal Name:
- Proceedings of the Royal Society B: Biological Sciences
- Volume:
- 291
- Issue:
- 2034
- ISSN:
- 0962-8452
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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