Sexual Selection and the Evolution of Egg-Sperm Interactions in Broadcast-Spawning Invertebrates

Abstract

Many marine invertebrate taxa are broadcast spawners, where multiple individuals release their gametes into the water for external fertilization, often in the presence of gametes from heterospecifics. Consequently, sperm encounter the considerable challenges of locating and fertilizing eggs from conspecific females. To overcome these challenges, many taxa exhibit species-specific attraction of sperm toward eggs through chemical signals released from eggs (sperm chemotaxis) and species-specific gamete recognition proteins (GRPs) that mediate compatibility of gametes at fertilization. In this prospective review, we highlight these selective forces, but also emphasize the role that sexual selection, manifested through sperm competition, cryptic female choice, and evolutionary conflicts of interest between the sexes (sexual conflict), can also play in mediating the action of egg chemoattractants and GRPs, and thus individual reproductive fitness. Furthermore, we explore patterns of selection at the level of gametes (sperm phenotype, gamete plasticity, and egg traits) to identify putative traits targeted by sexual selection in these species. We conclude by emphasizing the excellent, but relatively untapped, potential of broadcast-spawning marine invertebrates as model systems to illuminate several areas of research in post-mating sexual selection.

Chemically moderated gamete preferences predict offspring fitness in a broadcast spawning invertebrate

“…Thus, we can only speculate on the mechanisms by which sperm fine-tune their chemotactic responses to chemoattractants from different conspecific females. Previous speculation on this issue surround possible links between the control of chemoattractants governing gamete attraction and mechanisms underlying gamete recognition proteins that mediate sperm-egg interactions upon gamete contact (reviewed in [21]). In this way, egg-derived chemical cues may prevent sperm from needlessly tracking the chemoattractants from eggs with which they are ultimately incompatible.…”

“…Gamete concentrations for both sexes were standardized within and among blocks. Egg concentrations were calculated through extrapolation of observed counts within a known volume and adjusted to 2 Â 10 5 eggs ml 21 . Sperm density was estimated using an improved Neubauer haemocytometer and adjusted to 2 Â 10 6 sperm ml 21 .…”

“…Egg concentrations were calculated through extrapolation of observed counts within a known volume and adjusted to 2 Â 10 5 eggs ml 21 . Sperm density was estimated using an improved Neubauer haemocytometer and adjusted to 2 Â 10 6 sperm ml 21 . These gamete concentrations were determined during preliminary tests to ensure ceiling (100% fertilization rate) or basement (0% fertilization rate) effects did not hamper the interpretation of results.…”

“…Although traditionally explained as a mechanism for maximizing gamete encounter rates and ensuring conspecific fertilizations (reviewed in [21]), recent evidence suggests that sperm chemoattraction may also function to select compatible gametes. In the mussel Mytilus galloprovincialis, sperm that are simultaneously exposed to egg chemoattractants coming from two different females display consistent but differential 'preferences' for eggs that confer high fertilization rates in subsequent (nochoice) crosses [22].…”

“…This question is highly pertinent because biases in fertilization success do not always equate to biases in offspring survival and therefore gamete compatibility may not be predictive of offspring viability [23 -25]. Thus, an outstanding question is whether sperm chemoattraction moderates the selection of genetically compatible sperm that result in indirect fitness benefits for females [21].…”