Potential costs of heterospecific sexual interactions in golden orbweb spiders (Nephila spp.)

Gene expression and alternative splicing dynamics are perturbed in female head transcriptomes following heterospecific copulation

BACKGROUND

Despite the growing interest in the female side of copulatory interactions, the roles played by differential expression and alternative splicing mechanisms of pre-RNA on tissues outside of the reproductive tract have remained largely unknown. Here we addressed these questions in the context of con- vs heterospecific matings between Drosophila mojavensis and its sister species, D. arizonae. We analyzed transcriptional responses in female heads using an integrated investigation of genome-wide patterns of gene expression, including differential expression (DE), alternative splicing (AS) and intron retention (IR).

RESULTS

Our results indicated that early transcriptional responses were largely congruent between con- and heterospecific matings but are substantially perturbed over time. Conspecific matings induced functional pathways related to amino acid balance previously associated with the brain's physiology and female postmating behavior. Heterospecific matings often failed to activate regulation of some of these genes and induced expression of additional genes when compared with those of conspecifically-mated females. These mechanisms showed functional specializations with DE genes mostly linked to pathways of proteolysis and nutrient homeostasis, while AS genes were more related to photoreception and muscle assembly pathways. IR seems to play a more general role in DE regulation during the female postmating response.

CONCLUSIONS

We provide evidence showing that AS genes substantially perturbed by heterospecific matings in female heads evolve at slower evolutionary rates than the genome background. However, DE genes evolve at evolutionary rates similar, or even higher, than those of male reproductive genes, which highlights their potential role in sexual selection and the evolution of reproductive barriers.

Sexual Size Dimorphism: Evolution and Perils of Extreme Phenotypes in Spiders

Sexual size dimorphism is one of the most striking animal traits, and among terrestrial animals, it is most extreme in certain spider lineages. The most extreme sexual size dimorphism (eSSD) is female biased. eSSD itself is probably an epiphenomenon of gendered evolutionary drivers whose strengths and directions are diverse. We demonstrate that eSSD spider clades are aberrant by sampling randomly across all spiders to establish overall averages for female (6.9 mm) and male (5.6 mm) size. At least 16 spider eSSD clades exist. We explore why the literature does not converge on an overall explanation for eSSD and propose an equilibrium model featuring clade- and context-specific drivers of gender size variation. eSSD affects other traits such as sexual cannibalism, genital damage, emasculation, and monogyny with terminal investment. Coevolution with these extreme sexual phenotypes is termed eSSD mating syndrome. Finally, as costs of female gigantism increase with size, eSSD may represent an evolutionary dead end.

Frequent misdirected courtship in a natural community of colorful Habronattus jumping spiders

Male courtship display is common in many animals; in some cases, males engage in courtship indiscriminately, spending significant time and energy courting heterospecifics with whom they have no chance of mating or producing viable offspring. Due to high costs and few if any benefits, we might expect mechanisms to evolve to reduce such misdirected courtship (or ‘reproductive interference’). In Habronattus jumping spiders, males frequently court heterospecifics with whom they do not mate or hybridize; females are larger and are voracious predators, posing a severe risk to males who court indiscriminately. In this study, we examined patterns of misdirected courtship in a natural community of four sympatric Habronattus species (H. clypeatus, H. hallani, H. hirsutus, and H. pyrrithrix). We used direct field observations to weigh support for two hypotheses (differential microhabitat use and species recognition signaling) to explain how these species reduce the costs associated with misdirected courtship. We show that, while the four species of Habronattus do show some differences in microhabitat use, all four species still overlap substantially, and in three of the four species individuals equally encountered heterospecifics and conspecifics. Males courted females at every opportunity, regardless of species, and in some cases, this led to aggression and predation by the female. These results suggest that, while differences in microhabitat use might reduce misdirected courtship to some extent, co-existence of these four species may be possible due to complex communication (i.e. species-specific elements of a male’s courtship display). This study is the first to examine misdirected courtship in jumping spiders. Studies of misdirected courtship and its consequences in the field are limited and may broaden our understanding of how biodiversity is maintained within a community.