Abstract
Brachionus calyciflorus is a species complex consisting of four recently described species. Although several lines of evidence support their species status, hybridization between two of the sibling species B. calyciflorus s.s. and B. elevatus has been inferred from both field and laboratory studies. In this study, we tested for the existence of prezygotic barriers between these species by performing two types of cross-fertilization experiments. In a ‘mate competition’ experiment we exposed mictic females to equal numbers of conspecific and allospecific males and demonstrate that intraspecific fertilizations occur at much higher frequencies than interspecific fertilizations, providing evidence for a strong prezygotic reproductive barrier. This result was consistent across numerous combinations of parental genotypes. In addition, by creating interspecific combinations of mictic females and males in a ‘no choice’ experiment, we found that interspecific fertilization success is independent of fertilization direction while it does seem to depend on maternal genotype. Our results demonstrate the existence of a strong prezygotic barrier that may play an important role in the maintenance of species boundaries. Yet, the observation of hybrids also shows a potential for gene flow between the species through hybridization.
Highlights
The ‘species’ is a fundamental concept in taxonomy, ecology and evolution
Our results demonstrate the existence of a strong prezygotic barrier that may play an important role in the maintenance of species boundaries
With 1/24, the ratio of hybrid to intraspecific fertilizations was substantially lower than a 1:1 ratio, providing compelling evidence for the existence of a strong prezygotic barrier in both fertilization directions (P < 0.001)
Summary
The ‘species’ is a fundamental concept in taxonomy, ecology and evolution. there exist many definitions of this concept (de Queiroz, 2007; Leliaert et al, 2014), the biological species concept1 3 Vol.: (0123456789)(Dobzhansky, 1950; Mayr, 1942) is undoubtedly the most influential one. Mayr, (1942) defined the biological species as a group of interbreeding natural populations that are reproductively isolated from other such groups. Many studies suggest that prezygotic barriers contribute more to reducing interspecific gene flow than postzygotic barriers (Chin et al, 2019; Lackey & Boughman, 2017; Sánchez-Guillén et al, 2012). This is because prezygotic barriers are generally found to evolve more quickly (Sánchez-Guillén et al, 2014; Stelkens et al, 2010). Prezygotic barriers can effectively prevent waste of resources on unfavorable genomic combinations and thereby promote reproductive isolation of the parental species (Hopkins, 2013; Matute 2010). Elucidating how reproductive isolation is formed through different barriers is important for understanding the dynamics of speciation
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