Abstract
Male genitalia are usually extremely divergent between closely related species, but relatively constant within one species. Here we examine the effect of temperature on the shape of the ventral branches, a male genital structure involved in reproductive isolation, in the sister species Drosophila santomea and Drosophila yakuba. We designed a semi‐automatic measurement machine learning pipeline that can reliably identify curvatures and landmarks based on manually digitized contours of the ventral branches. With this method, we observed that temperature does not affect ventral branches in D. yakuba but that in D. santomea ventral branches tend to morph into a D. yakuba‐like shape at lower temperature. We found that male genitalia structures involved in reproductive isolation can be relatively variable within one species and can resemble the shape of closely related species’ genitalia through plasticity to temperature. Our results suggest that reproductive isolation mechanisms can be dependent on the environmental context.
Highlights
Phenotypic plasticity, the capacity for one genotype to generate multiple phenotypes in response to environmental variation, is a pervasive feature of biological systems (Debat & David, 2001; Klingenberg, 2019)
We previously reported that the shape of ventral branches in D. santomea, D. yakuba, and their hybrids can be characterized with a set of five manually detected landmarks, which allows to calculate via simple arithmetic how much the lateral spines rise above the central ridge, as a quantitative value named “spine thrust” (ST), expressed in micrometers (Peluffo et al, 2015)
We find here that this is true for D. yakuba but not for D. santomea: changing the rearing temperature from 25°C to 18°C leads to an increase in spine thrust in D. santomea male genitalia that is similar to what is observed between D. santomea and D. yakuba
Summary
Phenotypic plasticity, the capacity for one genotype to generate multiple phenotypes in response to environmental variation, is a pervasive feature of biological systems (Debat & David, 2001; Klingenberg, 2019). In order to analyze and quantify the possible link between plasticity, reproductive isolation, and interspecific divergence, we chose to examine the effect of temperature on a male primary sexual trait likely involved in reproductive isolation between two Drosophila sister species, D. santomea and D. yakuba. These two species form an attractive system because their natural environment is relatively well characterized, they are known to hybridize, and one of their most remarkable morphological differences is a primary sexual trait that seems to be involved in a “lock-and-key” mechanism. We used it to examine the plastic response of ventral branches development at 18°C and 25°C both in newly collected wild strains and in strains kept in the laboratory for many years
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