Dr. Rob Dillon, Coordinator

Thursday, August 11, 2005

Ampullariids Star at Asilomar

I am pleased to report that the annual meeting of the American Malacological Society, held in late June at the Asilomar Conference Center near Monterey, CA, was a great success. The registered attendance of about 150 well-assorted malacologists combined to present 120 papers and posters, including 10 on freshwater snails, as appended to the end of this message. Abstracts are available from the AMS web site:

The quality of the papers was generally excellent. And on Monday morning June 27, Yoichi Yusa of Nara Women’s University (yusa@cc.nara-wu.ac.jp) may have presented the most important research results I have ever heard in my 29 years of scientific meetings.

Although most mollusks are gonochoristic (sexes separate), great mystery has long surrounded molluscan mechanisms of sex determination. There have been a couple scattered reports of sex chromosomes in prosobranch gastropods. The research of Stan Allen, Ximing Guo and their colleagues in the 1990s suggested that sex determination in (partially protandric) oysters seems to be controlled by a single locus with a dominant male allele. But population sex ratios are often way off 1:1 in the Mollusca, and sometimes the bias may be attributable to differential growth or survivorship in the sexes, or partial protrandry, and sometimes it clearly isn’t.

At Asilomar Yusa described an impressive series of breeding experiments strongly suggesting that gender in Pomacea canaliculata is controlled by a small number of additive sex-determining genes, apparently scattered through the genome, inherited from both parents. Such an oligogenic sex determining mechanism has never before been suggested for the Mollusca. It seems clear to me that sex ratios might easily vary from 1:1 in this situation, especially in populations subject to drift and bottlenecks, such as many freshwater prosobranch snails. The evolutionary implications are profound.

While we’re on the subject of the Ampullariidae, I should also report that Ken Hayes (working with Rob Cowie at the University of Hawaii – Manoa, khayes@hawaii.edu) has been sequencing the daylights out of the family. He has (to this point) sampled somewhere around 9 – 13 Pomacea species from the Americas, as well as representatives of the genera Marisa, Asolene, Lanistes and Pila. His database currently includes about 435 individual CO1 sequences, from 40 populations in their native ranges and 80 introduced populations.

The big headline (from my outside perspective) is that Ken seems to find that sequence methods are useful in discriminating Pomacea species. Freshwater and terrestrial gastropods both typically show great intrapopulation sequence variation, to the point that the distinction between populations known to constitute valid biological species may be swamped. But Ken reports that the mean maximum intraspecific sequence divergence in his Pomacea data set is around 5%, while mean minimum interspecific divergence is around 10%, suggesting that sequence data may prove to be a useful tool for specific diagnosis in the Ampullariidae.

Although the various Pomacea species are not terribly difficult to culture, I don’t believe that Ken has breeding data of sufficient quality to absolutely confirm the biological status of his nominal species groups. Thus his sequence data, strictly speaking, remain uncalibrated. But he reassures me that anatomical morphology supports the specific distinctions being made by his CO1 sequences in all cases where they’ve looked. Regardless, it’s nice to see sequence data find some application not dependent on the tenuous assumptions of phylogenetic reconstruction.


Freshwater gastropod presentations at AMS 2005, Asilomar:
  • Robert T. Dillon, Jr., John D. Robinson, and Amy R. Wethington. Empirical Estimates of Reproductive Isolation Among the Freshwater Pulmonate Snails Physa acuta, P. pomilia, and P. hendersoni.
  • Kenneth A. Hayes. Preliminary phylogenetic assessment of invasive apple snails in Asia and beyond.
  • Cynthia G. Norton and Jennifer M. Bronson. The relationship between body size, growth, and egg production in the hermaphroditic freshwater snail, Helisoma trivolvis.
  • Robert S. Prezant and Eric J. Chapman. Temporal Community Structure and Biodiversity of Malacofauna from an Urban New Jersey Pond.
  • David C. Richards, C. Michael Falter, Gary T. Lester, Ralph Myers. Mollusk Survey and Basic Ecological Studies in Hells Canyon, Snake River, USA.
  • Ellen E. Strong. New morphological data for Pleuroceridae (Gastropoda, Cerithioidea): implications for monophyly and affinity of the family.
  • Andries Ter Maat, Cora Montagne-Wajer and Joris M. Koene. The year of the pond snail.
  • Lori Tolley-Jordan. Impacts if urbanization on the biodiversity of the imperiled snail fauna (Gastropoda: Prosobranchia: Pleuroceridae) of the Cahaba River, Alabama, USA.
  • A.R. Wethington, M.K. Smith, G. Oliveira, F. Lewis, and D.J. Minchella. Genetic Structure of Biomphalaria glabrata populations sampled from a schistosomiasis endemic region.
  • Yoichi Yusa. Genetics of Sex Ratio Variation in the Apple Snail, Pomacea canaliculata.