Monday, September 15, 2014

Genetic Distinction of the Owyhee wet-rock physa of southeastern Oregon

The freshwater gastropod family Physidae (Pulmonata: Basommatophora) has largely a Holarctic distribution, which extends southward into Central and South America (Burch, 1982, Taylor, 2003).  Physids, particularly Physa acuta have been introduced around the world.  Physidae diversity is concentrated in the United States and Canada, which has 47 species (Johnson et al. 2013).  Wethington and Lydeard (2007) were the first to examine the evolutionary relationships of the family using modern phylogenetic analyses of two mitochondrial genes of 66 specimens representing 28 taxa.  Wethington and Lydeard (2007) confirmed the recognition of a number of formal and informal taxonomic groups that had been identified by penial morphology (Te, 1978).  Although a complete systematic treatment is still needed for the family, the study at least provided an evolutionary framework that other studies could use as a starting point and/or an hypothesis of relationships that could be tested with additional data. 
Recently, an interesting paper entitled “Recognition of a highly restricted freshwater snail lineage (Physidae: Physella) in southeastern Oregon: convergent evolution, historical context, and conservation considerations” was published this year in Conservation Genetics by Alexandria Moore, John Burch and Thomas Duda, Jr.  The authors examined the phylogenetic status of the Owyhee wet-rock physa, which is restricted to a series of geothermal springs within the Owyhee River drainage in southeastern Oregon.  The Owyhee wet-rock physa has a shell-shape reminiscent of P. zionis rather than the typical physid shape exhibited by members of the gyrina species group, so P. zionis was included in the analysis as well (see photo above showing a) Owyhee wet-rock physa, b) P. zionis and c) P. cf gyrina).  Phylogenetic analyses of sequences of mitochondrial cytochrome oxidase I and two nuclear genes (internal transcribed spacer genes I and II), revealed the Owyhee wet-rock physa is genetically distinct and closely related to P. cf gyrina from California.  It is distantly related to P. zionis, so the similar shell shape evolved independently.  The authors plan to formally describe this unique species and recommend that it be considered critically endangered based on its limited distribution.  Clearly, physids offer a wealth of opportunity for those interested in sorting out evolutionary relationships, delimiting species boundaries and discovering taxa. 

Literature Cited
Burch, J. B.  1982.  North American freshwater snails: identification keys, generic synonymy, supplemental notes, glossary, references, index.  Walkerana, 1:1-365.
Johnson, P. D., A. E. Bogan, K. M. brown, N. M. Burkhead, J. R. Cordeiro, J. T. Garner, P. D. Hartfield, D. W. Lepitzki, G. L. Mackie, E. Pip, T. A. Tarpley, J. S. Tiemann, N. V. Whelan, and E. E. Strong.  2013.  Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38(6):247-282.
Moore, A. C., J. B. Burch, and T. F. Duda, Jr.  2014.  Recognition of a highly restricted freshwater snail lineage (Physidae: Physella) in southeastern Oregon: convergent evolution, historical context, and conservation considerations.  Conservation Genetics
Taylor, D. W.  2003.  Introduction to Physidae (Gastropoda: Hygrophila) biogeography, classification, morphology.  Revista de Biologia Tropical, Supplement 51:1-287.
Te, G. A.  1978.   The systematics of the family Physidae (Basommatophora: Pulmonata).  Ph.D. thesis, University of Michigan.
Wethington, A., C. Lydeard.  2007.  A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences.  J. Molluscan Studies 73:241-257.

Monday, August 11, 2014

Are Distributions of Ancient Lineages of Mollusks Shaped by Vicariance or Dispersal?

                It is thought that the study of animal and plant distributions advanced greatly with the development of vicariance biogeography in the 1970s and 1980s.  Rather than assuming related groups of organisms dispersed around the globe across seemingly insurmountable barriers, it was thought that organisms rafted apart from one another on drifting continents via plate tectonics yielding many groups that shared a common Earth history.  Vicariance biogeography revolutionized the discipline and stimulated many studies with an interest in constructing area cladograms and searching for generalized patterns.  Indeed, its influence was so profound that dispersal was considered less and less likely and some even believed it could not be falsified and therefore was unscientific.  Over the past decade, however, numerous studies have been published showing that the distribution of a number of taxonomic groups formerly thought to be due to vicariance are best explained by dispersal.  Indeed, one maverick, Alan de Queiroz, has proposed that most distributions can be attributed to dispersal.  He tells his very interesting story in a recently published book entitled The Monkey’s Voyage – How Improbable Journeys Shaped the History of Life.
                De Queiroz proposes a new paradigm for historical biogeography that entails three steps (1) acceptance of plate tectonics, (2) building evolutionary trees utilizing cladistic thinking or other statistical means, and the step that was missing in vicariance biogeography (3) time.  Time is an absolutely essential component and comes from molecular dating results.  So for example, the New World and Old World monkey molecular timeline is a split between 26 to 51 million years ago, which means they missed the vicariant event separating Africa from South America.  Therefore, Old World monkeys must have dispersed to South America.
                De Queiroz presents numerous case studies and makes a compelling case for dispersal shaping the history of life in a much more significant way than thought of during the vicariance revolution.  But is it possible, the pendulum will swing too far back from vicariance being dominant mechanism to dispersal without thoroughly investigating the matter?   For example, many of the case studies are vertebrate groups that simply are not old enough to play a part in many vicariant events including the ever popular break-up of Gondwana (although this has not stopped investigators from proposing vicariance during the vicariant movement).  However, de Queiroz cites a few exceptions involving two lineages of mite harvestmen (Giribet et al., 2012) and two lineages of centipedes (Murienne et al., 2010) that appear to be Gondwanan relicts dating back to 90 million years ago or earlier.  What about other ancient invertebrate groups?  What about mollusks?  It is known that dispersal plays a fundamental role in the evolution of biodiversity on oceanic islands including terrestrial gastropods (Cowie and Holland, 2006), but what about the break-up of continental landmasses like Gondwana?   Clearly, many mollusk lineages are ancient enough to possibly be shaped by vicariance, but studies are needed to determine whether this is the case or not.  I encourage malacologists to rise to this interesting challenge.

Cowie, R. H., and B. S. Holland.  2006.  Dispersal is fundamental to biogeography and the evolution of biodiversity on oceanic islands.  Journal of Biogeography 33:193-198.

Giribet, G., et al.  2012.  Evolutionary and biogeographical history of an ancient and global group of arachnids (Arachnida: Opiliones: Cyphophthalmi) with a new taxonomic arrangement.  Biological Journal of the Linnean Society 105:92-130.

De Queiroz, A.  2014.  The Monkey’s Voyage:  How Improbable Journeys Shaped the History of Life.  Basic Books, New York. 

Murienne, J., G. D. Edgecombe, and G. Giribet.  2010.  Including secondary structure, fossils and molecular dating in the centipede tree of life.  Molecular Phylogenetics and Evolution 57:301-313.

Friday, May 2, 2014

Towards a Bivalve Tree of Life

An important paper entitled “Investigating the Bivalve Tree of Life – an exemplar-based approach combining molecular and novel morphological characters” was recently published this year in Invertebrate Systematics.  This paper led by Rüdiger Bieler and Paula M. Mikkelsen and their team of 18 collaborators represents a significant product of their Bivalvia project (BivAToL), which is part of the Assembling the Tree of Life program sponsored by the National Science Foundation.  This paper, which uses an exemplar-based approach to generate morphological and molecular data from specimens from the same population or even the same specimen goes a long way towards providing the necessary phylogenetic infrastructure (sensu Bieler et al., 2013 and my April 2013 blog posting) for those interested in using bivalves as model organisms in a wide range of evolutionary and ecological studies. 

The BivAToL team examined stomach morphology and other features of the alimentary tract, the morphology of gills and labial palps, shell microstructure, and sperm ultrastructure.  Molecular markers included two nuclear ribosomal genes (18S and 28S rRNA), one nuclear protein encoding gene (histone H3), one mitochondrial ribosomal gene (16S rRNA) and one mitochondrial protein-encoding gene (cytochrome c oxidase subunit I; COI).  Four additional nuclear protein-encoding genes were also generated for a subset of taxa.  This impressive assemblage of data was analyzed using a variety of methods including parsimony, maximum likelihood and Bayesian inference with molecular data alone and in combination with the morphological characters.   The results consistently recovered the monophyly of Autobranchia, Pteriomorphia, Heterochonchia, Palaeoheterodonta, Archiheterodonta, Euheterodonta, Anomalodesmata and a new clade Imparidentia (=Euheterodonta excluding Anomalodesmata).  The authors also discussed the origin and diversification times for each of the major clades and provided a classification scheme for the six major monophyletic groups of modern Bivalvia.  This important paper is certain to stimulate much more research on bivalve systematics and and other interesting evolutionary questions.

Literature Cited

Bieler, R., P. M. Mikkelsen, T. M. Collins, E. A. Glover, V. L. González, D. L. Graf, E. M. Harper, J. Healy, G. y. Kawauchi, P. P. Sharma, S. Staubach, E. E. Strong, J. D. Taylor, I. Tȅmkin, J. D. Zardus, S. Clark, A. Guzmán, E. McIntyre, P. Sharp, and G. Giribet.  Investigating the bivalve tree of life – an exemplar –based approach combining molecular and novel morphological characters.  Invertebrate Systematics 28:32-115.

Bieler, R., P. M. Mikkelsen, and G. Giribet.  2013.  Bivalvia-A discussion of known unknowns.  American Malacological Bulletin 31(1):123-133.