Project Title:
Post-zygotic isolation in sympatric species of the sea urchins Echinometra and Lytechinus.
Mentor’s name:
Haris Lessios, Lessiosh@si.edu.
Location of internship
Galeta, Naos. Mentor at location.
Project summary
How do sympatric closely related species of sea urchins maintain their respective genetic identities? Inadequate knowledge of the origin and maintenance of reproductive isolation in such organisms that spawn externally and in which fertilization occurs in the water column remains one of the main obstacles of reconstructing the mode of their speciation. In the Caribbean there are two genera that contain sympatric species, Echinometra and Lytechinus. In both genera, there is some habitat separation between their respective species, but not enough to explain how their gametes avoid mixing. Annual and monthly reproductive cycles of Echinometra lucunter and E. viridis overlap and the same is true in Lytechinus williamsi and L. variegatus. In Lytechinus there is no preference of eggs for conspecific sperm, even though there is some divergence in the bindin molecule. In Echinometra divergence in bindin is pronounced and eggs of E. lucunter will not permit fertilization by sperm of E. viridis. The interspecific crosses in the other direction, however, proceeds almost as easily as intraspecific crosses, so, there is only one-way isolation. Thus, every potential pre-zygotic isolating barrier that we have studied today has failed to explain how congeneric species avoid hybridization. Yet, both isozyme and DNA data have confirmed that there is absolutely no introgression between the species of Echinometra. In Lytechinus natural hybrids can be found, but not in numbers that would suggest that the two species are merging. Morphology of each species in both genera is distinct. There is, therefore, a high probability that the isolating barrier is post-zygotic, and, given that hybrids are either absent or rare among post-metamorphic individuals, the post-zygotic barrier is likely to be acting at the larval stage. We are examining the possibility that hybrid larvae between E. lucunter and E. viridis and between L. variegatus and L. williamsi do not survive as well as non-hybrid larvae.
We have set up experiments of producing both hybrid and non- hybrid larvae and raising them to metamorphosis. Because many things can affect larval survivorship, hybrid and non-hybrid larvae from each set of parents are raised together in each jar. An aliquot of larvae is taken out of each jar every five days, and each larva is then genotyped to determine its parentage. To determine the parents of each larva in Echinometra we amplify a section of its bindin molecule where E. viridis has a large deletion relative to E. lucunter and use the size of the amplicon to tell us whether it is a hybrid or not. In Lytechinus, the same function is fulfilled with a set of microsatellites characteristic of each species.
Mentorship goals:
The intern will be helping in raising and genotyping the larvae. This work involves free-diving at Galeta to collect the adults, spawning them to obtain gametes, performing the crosses, then caring for the larvae daily until they metamorphose. Raising sea urchin larvae is an art, requiring a good feeling for the organism and an understanding of all things that can go wrong when larvae are grown in culture bottles. By the end of the internship, the intern will have learned techniques of raising larvae, and techniques for genotyping them. More important, the hope is that she/he will gain an appreciation of the means by which sympatric species of free-spawning marine organisms remain as separate species.
Desired background:
Interns should have interest in ecology and evolution of marine organisms and in learning molecular techniques. Most of the sampling is done by snorkeling, so interns should be competent swimmers and feel comfortable in working in the water.
Suggested background reading:
Rahman, M. A., T. Uehara, and J. S. Pearse. 2001. Hybrids of two closely related tropical sea urchins (Genus Echinometra): Evidence against postzygotic isolating mechanisms. Biol. Bull. 200:97-106. DOI:10.2307/1543303
K.S. Zigler, and H.A. Lessios. 2004b. Speciation on the coasts of the New World: Phylogeography, reproductive isolation, and the evolution of bindin in Lytechinus. Evolution 58:1225-1241. DOI: 10.1111/j.0014-3820.2004.tb01702.x
M.A. McCartney, G. Keller, H.A. Lessios. 2000a. Dispersal barriers in tropical oceans and speciation in Atlantic and eastern Pacific sea urchins of the genus Echinometra. Molecular Ecology 9:1391-1400. DOI 10.1046/j.1365-294x.2000.01022.x
