Selective forces that shape the evolution of gamete morphology are complex, and the links between gamete morphology, reproductive isolation and genetic divergence remain elusive. 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 egg accessory coats. A similar role has been suggested for sperm morphology but while basic sperm morphology has been described for many marine invertebrates, few data exist on within species variation in sperm traits, and the underlying genetic architecture remains to be examined. Unraveling the consequences and adaptive significance of within-species variation in sperm morphology and its pattern among free-spawning invertebrates may help elucidate the evolution of reproductive isolation, and thus the mechanisms that underlie the formation of new marine species.
Together with collaborators, I document among-population variation in
sperm morphology that is correlated with population divergence in neutral genetic
markers, and possibly with adaptive features of reproductive compatibility. This is
an expanding area, especially in echinoderms in which it's possible to assay variation
in gamete compatibility in vitro and parallel variation at some of the loci that encode
gamete recognition molecules. Combined with the use of microsatellites for paternity
analysis (allowing for experimental investigations into, for example, sperm precedence
among individuals and species), this is a tractable system for enquiries into the genetic
basis of species recognition, species-specific gamete traits and life-history evolution.
Comparing adaptive traits within and among species and populations from different
oceans and latitudes – from temperate to high arctic ecosystems, allows for results to
be interpreted in the context of adaptation to climate change, in addition to testing
theoretical evolutionary models.
work, I empirically test current theory on egg-size evolution and the role of
gamete traits and sperm availability. Here, we directly address an ongoing controversy
about the effect of egg-accessory coats on fertilization success and egg-size
evolution, and thus the evolution of anisogamy.
Strongylocentrotus droebachiensis shows spectacular intraspecific variation in egg size with latitude along the Norwegian coast, and variation in egg jelly-coat thickness that is inversely proportional to egg size. We use this geographic gradient to experimentally test the combined effects of egg and accessory-coat size on fertilization success. We show that the 'egg fertilizability' (the chance that a sperm will fertilize an egg if it encounters one) varies with target size and sperm concentration, and thus that the sea urchin system violates an assumption of the most widely used model of fertilization kinetics. We suggest the need to incorporate into existing fertilization kinetics models the interactions between egg size, total target size, and accessory-coat thickness, and how these vary with sperm concentration.
In free-spawning organisms, the risk of both incomplete fertilization (related to sperm limitation) and lethal fertilization by more than one sperm (under conditions of sperm competition) may influence the evolution of traits related to fertilization success. Given increasing evidence for great environmental heterogeneity in sperm concentration during natural spawning events, we provide evidence that studies of fertilization dynamics in free-spawning organisms should consider the role of egg-accessory coats in terms of a possible tradeoff between egg size and total target size and its complicated relationship to sperm.
Marks, J.A., Biermann, C.H., Earnes, W.F. and H. Kryvi. 2008. Sperm polymorphism within the sea urchin strongylocentrotus droebachiensis: Divergence between Pacific and Atlantic oceans. Biol. Bull. 215: 115-125. pdf
Biermann, C. H., Marks, J. A., Vilela-Silva, A-C. E. S.- Castro, M. O. and P. A. S. Mourão. 2004. Carbohydrate-based species recognition in sea urchin fertilization: another avenue for speciation? Evolution and Development 6(5): 353-361. pdf
Photo: C. Biermann
Understanding marine population connectivity is critical for sustainable management of marine resources. The degree to which populations of marine organisms exchange migrants determines whether they function as one large metapopulation or many independent units and thus influences their response to potential disturbance from harvesting, habitat destruction or climate change. As such, knowledge of connectivity directly impacts management decisions and addresses the question of whether fisheries management and the design of marine protected areas should be tailored to variation in coastal geography and regional oceanographic conditions.
It is often assumed that marine species with long-lived planktonic larvae disperse great distances in ocean currents and will thus exhibit low levels of genetic differentiation. However, recent evidence shows that significant population-genetic subdivision may occur even in species with a long larval duration and that variability in ocean currents can influence the spatial genetic structure of marine populations.
We are presently working to determine the population
genetic structure of the sea urchins Strongylocentrotus
droebachiensis and Strongylocentrotus
pallidus along the Norwegian coast and
The common green sea urchin, Strongylocentrotus droebachiensis has a circumarctic distribution,
and shows substantial genetic subdivision between northeastern Atlantic
populations and northwestern
Throughout their geographic range, these urchins
exhibit differences in life-history traits including egg size, larval duration
and size at metamorphosis (Biermann et al. 2004; Marks, unpublished data). Egg
size increases 4-fold in volume along a latitudinal gradient from
In many marine fish, both adults and larvae contribute
to migration among populations. Sea urchins have benthic adults with relatively
little mobility, and population genetic structure will be determined by the
interaction of regional oceanographic conditions and life-history traits. As
such, the sea urchin system provides a proxy for dispersal of other commercially
important organisms with pelagic larvae along the Norwegian coast and
Marks, J.A., Biermann, C.H., Earnes, W.F. and H. Kryvi. 2008. Sperm polymorphism within the sea urchin strongylocentrotus droebachiensis: Divergence between Pacific and Atlantic oceans. Biol. Bull. 215: 115-125. pdfBiermann, C. H. and J. A. Marks. 2000. Geographic divergence of gamete recognition systems in two species of the sea urchin genus Strongylocentrotus. Zygote 8 (S):86-87. pdf
Marks, J.A. 1993. Systematics and ecology of three didemnid ascidians: Didemnum albidum (Verrill, 1871), Didemnum polare (Hartmeyer, 1903) and Didemnum romssae, new species. Cand.scient. thesis, University of Tromsø, Norway, pp. 89.
Marks, J.A., 1996. Three sibling species of subarctic didemnid ascidians: Didemum albidum (Verrill, 1871), Didemnum polare (Hartmeyer, 1903) and Didemnum romssae, new species. Can. J. Zool. 74:357-379. pdf
Marks, J.A. (unpublished ms.). Contrasting overwintering in two sibling species of arctic marine invertebrate.
Marks, J.A. (ms. in prep. for FishfarmingXpert magazine). Alien sea quirt Didemnum vexillum: invasive tunicate species threatens fish-farming industry.
Padilla, D.K., Harvell, C.D., Marks, J.A. and B. Helmuth. 1996. Induceible aggression and intraspecific competition for space in a marine bryozoan, Membranipora membranecea. Limnol. Oceanog. 41(3):505-512.
Vermeij, G.J., R. B. Lowell, L. J. Walters and J. A. Marks. 1987. Good hosts and their guests: Relations between trochid gastropods and the epizoic limpet Crepidula adunca. The Nautilus. 101(2):69-74.