Ejaculate Allocation and Sperm Characteristics Differ among Alternative Male Types in a Species of Fish with Cooperation and Competition among Unrelated Males.

Suzanne H Alonzo,Kelly A Stiver,Holly K Kindsvater,Susan E Marsh-Rollo,Erem Kazancıoğlu,Bridget Nugent

doi:10.3390/cells10102612

Abstract

Sexual selection arising from sperm competition has driven the evolution of immense variation in ejaculate allocation and sperm characteristics not only among species, but also among males within a species. One question that has received little attention is how cooperation among males affects these patterns. Here we ask how male alternative reproductive types differ in testes size, ejaculate production, and sperm morphology in the ocellated wrasse, a marine fish in which unrelated males cooperate and compete during reproduction. Nesting males build nests, court females and provide care. Sneaker males only “sneak” spawn, while satellite males sneak, but also help by chasing away sneakers. We found that satellite males have larger absolute testes than either sneakers or nesting males, despite their cooperative role. Nesting males invested relatively less in testes than either sneakers or satellites. Though sneakers produced smaller ejaculates than either satellite or nesting males, we found no difference among male types in either sperm cell concentration or sperm number, implying sneakers may produce less seminal fluid. Sperm tail length did not differ significantly among male types, but sneaker sperm cells had significantly larger heads than either satellite or nesting male sperm, consistent with past research showing sneakers produce slower sperm. Our results highlight that social interactions among males can influence sperm and ejaculate production.

Highlights

We found that satellite males had significantly greater variation in testes weight than either sneaker males
We found differences among alternative male types in absolute testes size, the allometric relationship between testes and body size, ejaculate production, and sperm cell morphology
We argue that further study of male social roles and interactions may help explain some of this variation, especially in species with alternative male types that differ in sperm competition and in their social roles and energy budgets

Summary

Introduction

Sperm competition drives sexual selection on a variety of male traits, including the amount of energy males allocate to sperm production, the type of sperm they produce, and the quantity of and compounds found in their seminal fluid [1,2,3,4,5,6,7,8,9]. Sperm cells are one of the most diverse and rapidly evolving cells in the animal kingdom [10,11], and divergence in seminal fluid proteins contributes to reproductive isolation and speciation and biodiversity [12,13,14]. Releasing more sperm and producing faster sperm generally increases fertilization success [32,33]; but see [34]

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