Abstract

Competition between same-sex organisms, or intra-sexual selection, can occur before and after mating, and include processes such as sperm competition and cryptic female choice. One of the consequences of intra-sexual selection is that male reproductive traits tend to evolve and diverge at high rates. In benthic octopuses, females often mate with more than one male in a single reproductive event, opening the venue for intra-sexual selection at multiple levels. For instance, males transfer spermatophores through hectocotylus, and can remove the spermatophores left by other males. Considering the limited evidence on post-copula competition in benthic octopuses, and the potential to affect the evolution of reproductive traits within octopodids, we put this hypothesis to a test employing a phylogenetic comparative approach. We combined data on hectocotylized arm length (HAL), ligula length (LL), spermatophore length (SL) with a Bayesian molecular phylogeny of 87 species, to analyze how reproductive traits have diverged across lineages and covary with body size (mantle length; ML). First, additionally to ML, we estimated the phylogenetic signal (λ) and mode of evolution (κ) in each reproductive trait. Second, we performed phylogenetic regressions to quantify the association among reproductive traits and their co-variation with ML. This analysis allowed us to estimate the phenotypic change along a branch into the phylogeny, and whether selection may have played a role in the evolution and diversification of specific clades. Estimations of λ were always high (>0.75), indicating concordance between the traits and the topology of the phylogenetic tree. Low values of κ (<1.0) suggested that evolution depends on branch lengths. All reproductive traits exhibiting a positive relation with ML (β > 0.5 in all cases). Overall, evolutionary rate models applied to the SL-ML regression suggested that octopuses of the family Megaleledonidae have evolved larger spermatophores than expected for their size. The regression HAL-ML indicated that HAL was more variable in Megaleledonidae than in the remaining clades, suggesting that the high divergence across species within this group might partially reflect intra-sexual selection. These results support the hypothesis that, at least in some lineages, sexual selection may account for the divergence in reproductive traits of male octopuses.

Highlights

  • The evolution of mate choice and mating competition has been a major component of Darwin’s theory of sexual selection (Darwin, 1871)

  • Regarding the mode of evolution inferred by κ, calculated in combination with λ in the univariate analyses, estimates for mantle length (ML), hectocotylized arm length (HAL), ligula length (LL), and spermatophore length (SL) were intermediate between κ = 0 and 1 and statistically different from those values according to Bayes Factor (BF) estimates (BF > 1.39 for all comparisons) (Table 2)

  • The present results support our original hypothesis on reproductive traits in male benthic octopuses, evidencing that spermatophores, and hectocotyli exhibited accelerated rates of evolution, at least in several Antarctic and deep-water lineages (Megaleledonidae and Cistopus), presumably due to sexual selection

Read more

Summary

Introduction

The evolution of mate choice and mating competition has been a major component of Darwin’s theory of sexual selection (Darwin, 1871). Because of its relevance to males’ reproductive success (Eberhard, 1998; Snook, 2005), multiple responses have evolved to outcompete rival males, including: (i) the production of spermatophores or packages of sperm (Mann, 1984; Nigmatullin et al, 2003) that, when transferred to females, may occupy considerable space within the storage organs preventing other spermatophores from being stored (Thornhill and Alcock, 1983), (ii) the removal of other males’ spermatophores during copulation (Cigliano, 1995), or (iii) the production of sperm with inhibitory effects on the rival males’ sperm function (Snook, 2005) Because of this evolutionary arms race, the genitalia of several organisms exhibit extreme differences in size and shape across closely related species and are presumed to evolve faster than other traits (Eberhard, 1985; Genevcius et al, 2017)

Methods
Results
Conclusion
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call