Variation in inbreeding depression within and among Caenorhabditis species

TL;DR

The study explores how inbreeding affects fitness in different species of Caenorhabditis worms, finding that inbreeding depression varies widely and may not always prevent the evolution of self-fertilization.

Contribution

The paper demonstrates that inbreeding depression varies significantly among and within species of obligately outcrossing Caenorhabditis nematodes.

Findings

Inbreeding depression is universal but highly variable among species and populations of Caenorhabditis.

Inbreeding depression affects multiple life stages, including mating, embryo production, and larval growth.

Some species, like Caenorhabditis becei, show modest inbreeding depression and could serve as laboratory models.

Abstract

Outbreeding populations harbor large numbers of recessive deleterious alleles that reduce the fitness of inbred individuals, and this inbreeding depression potentially shapes the evolution of mating systems, acting as a counterweight to the inherent selective advantage of self-fertilization. The population biological factors that influence inbreeding depression are numerous and often difficult to disentangle. We investigated the utility of obligately outcrossing Caenorhabditis nematodes as models for inbreeding depression. By systematically inbreeding lines from 10 populations and tracking line extinction, we found that inbreeding depression is universal but highly variable among species and populations. Inbreeding depression was detected across the life cycle, from mating to embryo production to embryonic viability and larval growth, and reciprocal crosses implicated female-biased effects. In most cases, the surviving inbred lines have dramatically reduced fitness, but the variance among inbred lines is substantial and compatible with the idea that inbreeding depression need not be an obstacle to the evolution of selfing in these worms. Populations of some species, including Caenorhabditis becei, exhibited modest inbreeding depression and could be tractable laboratory models for obligately outcrossing Caenorhabditis.

A lot of genetic variation is harmful and populations harbor different amounts of it. Recessive variation is a particularly important part of this story, as it can persist invisibly in outbreeding populations. Rockman et al. show that outbreeding relatives of C. elegans have lots of recessive deleterious variation, but species differ in the amount, and so do isolates within species. Highly inbred lines derived from these different sources vary in their fitness, and in some cases the fitness is high enough that an evolutionary transition from outcrossing to selfing would not be impeded by inbreeding depression.

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