Abstract
Many parasitoids have single‐locus complementary sex determination (sl‐CSD), which produces sterile or inviable males when homozygous at the sex determining locus. A previous study theoretically showed that small populations have elevated risks of extinction due to the positive feedback between inbreeding and small population size, referred to as the diploid male vortex. A few modeling studies have suggested that the diploid male vortex may not be as common because balancing selection at sex determining loci tends to maintain high allelic diversity in spatially structured populations. However, the generality of the conclusion is yet uncertain, as they were drawn either from models developed for particular systems or from a general‐purpose competition model. To attest the conclusion, we study several well‐studied host–parasitoid models that incorporate functional response specifying the number of attacked hosts given a host density and derive the conditions for a diploid male vortex in a single population. Then, we develop spatially structured individual‐based versions of the models to include female behavior, diploid male fertility, and temporal fluctuations. The results show that producing a handful of successful offspring per female parasitoid could enable parasitoid persistence when a typical number of CSD alleles are present. The effect of functional response depends on the levels of fluctuations in host abundance, and inviable or partially fertile diploid males and a small increase in dispersal can alleviate the risk of a diploid male vortex. Our work supports the generality of effective genetic rescue in spatially connected parasitoid populations with sl‐CSD. However, under more variable climate, the efficacy of the CSD mechanism may substantially decline.
Highlights
Small populations are susceptible to inbreeding and reduced population growth rate caused by the positive feedback between small population size and inbreeding depression, known as the extinction vortex (Gilpin & Soule, 1986)
This positive feedback may occur in a more exacerbated form in hymenopteran taxa, which have the haplodiploid genetic system with complementary sex determination (CSD; Heimpel & de Boer, 2008). In these taxa, unfertilized haploid eggs develop into males, while fertilized diploid eggs develop into females only if they are heterozygous at the CSD loci (Figure 1a; Heimpel & de Boer, 2008)
Previous modeling studies have shown that a diploid male vortex may not occur as readily in nature because migration and negative frequency-dependent selection can maintain allelic variation at the CSD loci in spatially structured populations (Hein et al, 2009; Nair et al, 2018) and because coupled dynamics of host and parasitoid populations dampen unstable oscillations (Bompard et al, 2016)
Summary
Small populations are susceptible to inbreeding and reduced population growth rate caused by the positive feedback between small population size and inbreeding depression, known as the extinction vortex (Gilpin & Soule, 1986) This positive feedback may occur in a more exacerbated form in hymenopteran taxa (sawflies, bees, wasps, and ants), which have the haplodiploid genetic system with complementary sex determination (CSD; Heimpel & de Boer, 2008). Previous modeling studies have shown that a diploid male vortex may not occur as readily in nature because migration and negative frequency-dependent selection can maintain allelic variation at the CSD loci in spatially structured populations (Hein et al, 2009; Nair et al, 2018) and because coupled dynamics of host and parasitoid populations dampen unstable oscillations (Bompard et al, 2016). This study suggests that the maintenance of habitat connectivity should be an effective conservation strategy for parasitoid populations with sl-CSD in fragmented landscapes under anticipated more extreme and variable climate
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