Cover
The male specimen of the common green sea urchin Strongylocentrotus droebachiensis shown on the cover was caught in the act of releasing sperm. Jessica Marks and her colleagues employed this species to investigate how the morphology and ultrastructure of sperm vary among different populations of green sea urchins and how closely related these populations are to each other genetically.
This question of intraspecific variation in sperm traits is significant because it sheds light on the complex selective forces that shape the evolution of intraspecific gamete morphology, which, in turn, may help us understand the evolution of reproductive isolation and, ultimately, how new marine species are formed and maintained.
S. droebachiensis is a good candidate for this research, described on pages 115 to 125, because it is among the most widely distributed echinoderm species and shows significant genetic divergence between oceans and populations. The urchin shown here is from Tromso, Norway, but the species occurs in northern seas in both the Atlantic and Pacific oceans and extending into the Arctic. Its habitat ranges from the intertidal zone down to a depth of 1000 m or more.
In marine invertebrates, positive selection on reproductive traits is thought to drive the rapid divergence of sperm and egg proteins. Likewise, sexual selection has been implicated in the evolution of egg size, and a similar role has been suggested for sperm morphology. However, although basic sperm morphology has been described for many marine invertebrates, few data exist on population-level variation in sperm traits or their underlying genetic architecture. In this issue, Marks et al. describe intraspecific variation in sperm morphology and ultrastructure from four populations of S. droebachiensis from two oceans. They also describe among-population variation in the amount of filamentous actin present prior to the acrosome reaction by which a sperm prepares itself to penetrate an egg. The results showed that not only does sperm morphology vary among populations, but sperm structure and genetic affiliation are correlated on a large geographic scale: Pacific and northwest Atlantic populations group together, while Arctic and northeast Atlantic populations show greater similarity in sperm shape and size, reflecting patterns of divergence in both mitochondrial DNA and the nuclear gene for sperm bindin, the protein that attaches sperm to eggs.
Credits:Photo:Erling Svensen(http://www.uwphoto.no). Cover design, Beth Liles (Marine Biological Laboratory).