anisogamy

dustinmeeg

Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm

Authors: Keyne Monro and Dustin J Marshall

Published in: Proceedings of the Royal Society B, volume 283, issue 1834 (July 2016)

Abstract

Gamete dimorphism (anisogamy) defines the sexes in most multicellular organisms.

Theoretical explanations for its maintenance usually emphasize the size-related selection pressures of sperm competition and zygote survival, assuming that fertilization of all eggs precludes selection for phenotypes that enhance fertility. In external fertilizers, however, fertilization is often incomplete due to sperm limitation, and the risk of polyspermy weakens theadvantage of high sperm numbers that is predicted to limit sperm size, allowing alternative selection pressures to target free-swimming sperm.

We asked whether egg size and ejaculate size mediate selection on the free-swimming sperm of Galeolaria caespitosa, a marine tubeworm with external fertilization, by comparing relationships between sperm morphology and male fertility across manipulations of egg size and sperm density.

Our results suggest that selection pressures exerted by these factors may aid the maintenance of anisogamy in external fertilizers by limiting the adaptive value of larger sperm in the absence of competition. In doing so, our study offers a more complete explanation for the stability of anisogamy across the range of sperm environments typical of this mating system and identifies new potential for the sexes to coevolve via mutual selection pressures exerted by gametes at fertilization.

Citation

Monro K, Marshall DJ (2016) Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm, Proceedings of the Royal Society B, 283:1834 PDF 2.7 MB doi:10.1098/rspb.2016.0671

dustinmeeg

The maintenance of sperm variability: context-dependent selection on sperm morphology in a broadcast spawning invertebrate

Authors: Darren W Johnson, Keyne Monro, and Dustin J Marshall

Published in: Evolution, volume 67, issue 5 (May 2013)

Abstract

Why are sperm so variable despite having a singular, critical function and an intimate relationship with fitness?

A key to under-standing the evolution of sperm morphology is identifying which traits enable sperm to be successful fertilizers. Several sperm traits (e.g., tail length, overall size) are implicated in sperm performance, but the benefits of these traits are likely to be highly con- text dependent.

Here, we examined phenotypic selection on sperm morphology of a broadcast spawning tube worm (Galeolaria gemineoa). We conducted laboratory experiments to measure the relationship between average sperm morphology and relative fertilization success across a range of sperm environments that were designed to approximate the range of sperm concentrations and ages encountered by eggs in nature.

We found that the strength and form of multivariate selection varied substantially across our environmental gradients. Sperm with long tails and small heads were favored in high-concentration environments, whereas sperm with long heads were favored at low concentrations and old ages.

We suggest variation in the local fertilization environment and resulting differences in selection can preserve variability in sperm morphology both within and among males.

Full paper

Johnson D, Monro K, Marshall DJ (2013) The maintenance of sperm variability: context-dependent selection on sperm morphology in a broadcast spawning invertebrate. Evolution, 67-5: 1383–1395 PDF PDF 889 KB doi:10.1111/evo.12022