Testing the Role of Natural and Sexual Selection on Testes Size Asymmetry in Anurans

Abstract

Directional asymmetry in testes size is commonly documented in vertebrates. The degree of testes size asymmetry has been confirmed to be associated with natural and sexual selection. However, the role of natural and sexual selection driving variations in testes size asymmetry among species of anurans are largely unknown. Here, we studied the patterns of variations in testes size asymmetry and the factors shaping its variations among 116 anuran species. The results indicated that the left size-biased testes in 110 species (94.83% of 116 species) is more common than the right size-biased testes in six species. For all studied species, the degree of testes size asymmetry was positively associated with relative livers and body fat mass, following the prediction of the packaging hypothesis. We also found that the postcopulatory sperm competition (e.g., residual testes size) was positively associated with the degree of testes asymmetry. However, environmental stress (e.g., high latitude, precipitation seasonality and temperature seasonality) did not promote more symmetrical testes for all species. Our findings suggest that both natural selection for larger livers in body space and sexual selection for rapid increase in testis mass for most species during the breeding season can play key roles in driving in testes size asymmetry across anuran species.

Keywords: anurans; body size; environmental stress; natural selection; sexual selection; testes size asymmetry.

Figure 1

Distribution map of sampling site.

Figure 2

Phylogenetic tree of the 116 anuran species based on the three nuclear genes (RAG1, RHOD and TYR) and the six mitochondrial genes (CYTB, 12S, 16S, COI, ND2 and ND4) using BEAUTi and BEAST v.1.8.3. Blue histogram indicates the degree of testes size asymmetry.

Figure 3

Partial regression plot of residual testes size asymmetry on residual combined testes mass when controlling for phylogeny and SVL across 116 species of anurans. SVL and combined testes mass were log₁₀-transformed. Species with higher residual combined testes mass are expected to encounter more intense sperm competition.

Figure 4

Partial regression plot of residual testes size asymmetry on residual livers mass when controlling for effects of combined testes mass, SVL and phylogeny across 116 species of anurans. SVL, combined testes mass and livers mass were log₁₀-transformed.