Phylogeny suggests nondirectional and isometric evolution of sexual size dimorphism in argiopine spiders

Abstract

Sexual dimorphism describes substantial differences between male and female phenotypes. In spiders, sexual dimorphism research almost exclusively focuses on size, and recent studies have recovered steady evolutionary size increases in females, and independent evolutionary size changes in males. Their discordance is due to negative allometric size patterns caused by different selection pressures on male and female sizes (converse Rensch's rule). Here, we investigated macroevolutionary patterns of sexual size dimorphism (SSD) in Argiopinae, a global lineage of orb-weaving spiders with varying degrees of SSD. We devised a Bayesian and maximum-likelihood molecular species-level phylogeny, and then used it to reconstruct sex-specific size evolution, to examine general hypotheses and different models of size evolution, to test for sexual size coevolution, and to examine allometric patterns of SSD. Our results, revealing ancestral moderate sizes and SSD, failed to reject the Brownian motion model, which suggests a nondirectional size evolution. Contrary to predictions, male and female sizes were phylogenetically correlated, and SSD evolution was isometric. We interpret these results to question the classical explanations of female-biased SSD via fecundity, gravity, and differential mortality. In argiopines, SSD evolution may be driven by these or additional selection mechanisms, but perhaps at different phylogenetic scales. K E Y W O R D S :Argiope, Brownian motion, comparative method, phylogenetically independent contrasts, Rensch's rule, size evolution.

Disentangling the Size and Shape Components of Sexual Dimorphism

“…This lineage consists of approximately 100 species (ca. 83 in the genus Argiope, 13 Gea and 3 Neogea) and these have been shown to differ dramatically in their sizes and in SSD (Cheng and Kuntner 2014). In this study, we examined 656 specimens of 47 species from all three genera for sizes and shapes (Table S1).…”

“…In the orb web spider subfamily Argiopinae, the sexes differ not only in body size (Cheng and Kuntner 2014) but also in abdomen shape with females exhibiting greater diversity. The female abdomens may be oval (Gea, Neogea and some Argiope), long and cylindrical (Argiope ocula), pentagonal (A. aetherea and A. appensa) or lobed (A. lobata and A. blanda) (Bjørn 1997;Levi 1983Levi , 2004.…”

“…Here, we examined argiopine intra-specific SShD and inter-specific abdominal morphotypes, reconstructed the evolution of abdominal shape in both sexes using a recent species level phylogeny (Cheng and Kuntner 2014), investigated the relationship between body size and different morphotypes and tested the correlation between sexual size and shape dimorphism. We also investigated whether female abdominal morphotypes and sizes reflect differences in abdominal weight, used as a proxy for abdomen volume (that, in turn, indicative of fecundity).…”

Cryptic Female Choice Within the Genus Argiope: A Comparative Approach

“…Comparative data are slowly accumulating and can soon be analyzed using a comparative approach based on a phylogenetic hypothesis (Cheng and Kuntner 2014). In this book chapter, we presented evidence for cryptic female choice defined as variation in paternity in several stages of the mating process, starting with the duration of copulation, over the application of a plug and the potential of re-mating as well as selective sperm storage.…”

“…Under laboratory conditions, females showed this acceptance behavior almost invariably suggesting that male courtship behavior elicits an immediate reflex action at least in females that were individually laboratory-reared and thus deprived of potentially important information on population density and the availability of mating partners. The scapepossessing species A. bruennichi and A. aurantia are sister species as well as derived from non-scape-possessing ancestors according to a recent phylogenetic hypothesis by (Cheng and Kuntner 2014) and both species mate in the hub of the female web.…”