Correlation between investment in sexual traits and valve sexual dimorphism in Cyprideis species (Ostracoda)

Abstract

Assessing the long-term macroevolutionary consequences of sexual selection has been hampered by the difficulty of studying this process in the fossil record. Cytheroid ostracodes offer an excellent system to explore sexual selection in the fossil record because their readily fossilized carapaces are sexually dimorphic. Specifically, males are relatively more elongate than females in this superfamily. This sexual shape difference is thought to arise so that males carapaces can accommodate their very large copulatory apparatus, which can account for up to one-third of body volume. Here we test this widely held explanation for sexual dimorphism in cytheroid ostracodes by correlating investment in male genitalia, a trait in which sexual selection is seen as the main evolutionary driver, with sexual dimorphism of carapace in the genus Cyprideis. We analyzed specimens collected in the field (C. salebrosa, USA; C. torosa, UK) and from collections of the National Museum of Natural History, Washington, DC (C. mexicana). We digitized valve outlines in lateral view to obtain measures of size (valve area) and shape (elongation, measured as length to height ratio), and obtained several dimensions from two components of the hemipenis: the muscular basal capsule, which functions as a sperm pump, and the section that includes the intromittent organ (terminal extension). In addition to the assessment of this primary sexual trait, we also quantified two dimensions of the male secondary sexual trait—where the transformed right walking leg functions as a clasping organ during mating. We also measured linear dimensions from four limbs as indicators of overall (soft-part) body size, and assessed allometry of the soft anatomy. We observed significant correlations in males between valve size, but not elongation, and distinct structural parts of the hemipenis, even after accounting for their shared correlation with overall body size. We also found weak but significant positive correlation between valve elongation and the degree of sexual dimorphism of the walking leg, but only in C. torosa. The correlation between the hemipenis parts, especially basal capsule size and male valve size dimorphism suggests that sexual selection on sperm size, quantity, and/or efficiency of transfer may drive sexual size dimorphism in these species, although we cannot exclude other aspects of sexual and natural selection.

The evolution of sexual dimorphism in gene expression in response to a manipulation of mate competition

“…Mating systems vary widely among species and, hence, this variation is presumed to be a major source of variation in sexual dimorphism, though this has received limited direct attention (Fernandes Martins et al, 2017). At the expression level, Harrison et al (2015) used a comparative approach to examine the effect of mating system on variation in dimorphism among six bird species.…”

Shifts in sexual dimorphism across a mass extinction in ostracods: implications for sexual selection as a factor in extinction risk

“…This shape difference arises from an expansion of the posterior region in the mineralized shell that accommodates the large sperm pump and copulatory apparatus in males [28]. In living ostracod species, the degree of dimorphism can be taken as an indicator of the intensity of sexual selection, with larger and more elongate males indicating greater male investment in reproduction [29]. In a previous study examining background extinction in the Late Cretaceous, we reported that increased extinction risk was associated with strong size and shape dimorphism that reflect high investment in male sexual traits [30].…”