Wednesday, November 26, 2014

Redescription and Description of Threatened Thiarids (Gastropoda: Cerithioidea) from Argentina-Paraguay

A. brunneum holotype

Although knowledge of the composition and phylogenetic placement of Thiaridae has increased due to recent studies (Strong et al., 2011 and citations therein), modern taxonomic and systematic treatment is still needed within the family.  Recently, Roberto Vogler, Ariel Beltramino, Juana Peso, and Alejandra Rumi provided a redescription of one species and a description of a new species from Argentina and Paraguay in an attempt to provide useful data towards future comparative studies.  The paper entitled “Threatened gastropods under the evolutionary genetic species concept: redescription and new species of the genus Aylacostoma (Gastropoda: Thiaridae) from High Paraná River (Argentina-Paraguay)” begins with an introduction to thiarids and a history of available genus-group names that are available for Neotropical thiarids.  The authors describe that in Argentina and Paraguay three species of Aylacostoma were recorded in the High Paraná River at the border area between Argentina and Paraguay, but regrettably, their highly oxygenated habitat disappeared due to the construction of a dam that formed the Yacyretá Reservoir.  The authors suggest that Aylacostoma guaraniticum and A. stigmaticum could now be categorized as extinct while A. chloroticum and the newly described species exist due to an ex situ conservation program.

Vogler et al. examined specimens from the Museo de La Plata and Museo Argentino de Ciencias Naturales and live specimens from the ongoing ex situ conservation program taking place at the Universidad Nacional de Misiones.  They took seven shell measurements and examined the protoconchs, operculate, and radulae using scanning electron microscopy.  They also obtained mitochondrial Cyt b sequences from five individuals and conducted phylogenetic analyses of mitochondrial COI data of 37 A. chloroticum and six A. new sp. as well as six outgroup taxa.    

The authors used the Evolutionary Genetic Species Concept to determine if well-supported sister clades could be different species.  The method, which has been developed for asexual species states that if phylogenetic analysis shows that small samples from two populations are reciprocally monophyletic , and if the mean sequence difference between them is more than four times θ=2Neµ estimated from the within-sample variation, then the samples came from different species (Birky et al., 2010).  The authors found support for the recognition of the redescribed A. chloroticum and new species, which they named A. brunneum Vogler & Peso.  Aylacostoma brunneum could also be distinguished from A. chloroticum by its distinctive coloration pattern being dark brown with alternating lighter brown bands, while A. cloroticum is greenish-yellow to mid greenish-brown.

Unfortunately, only a single relict population of A. chloroticum is known in the wild, and wild populations of A. brunneum are probably extinct (although further survey work is necessary).  It is a sad state of affairs that as scientists begin to gain knowledge of this poorly studied group, they are disappearing from the wild.
Literature Cited
Birky, C. W., Jr., J. Adams, M. Gemmel, and J. Perry.  2010.  Using population genetic theory and DNA sequences for species detection and identification in asexual organisms.  PLoS ONE 5: e10609. Doi:10.1371/journal.pone.0010609.

Strong, E. E., D. J. Colgan, J. M. Healy, C. Lydeard, W. F. Ponder, and M. Glaubrecht.  2011.  Phylogeny of the gastropod superfamily Cerithioidea using morphology and molecules.  Zool. J. Linnean Society 162:43-89.

Vogler, R. E., A. A. Beltramino, J. G. Peso, and A. Rumi.  Threatened gastropods under the evolutionary genetic species concept: redescription and new species of the genus Aylacostoma (Gastropoda: Thiaridae) from High Parana River (Argentina-Paraguay).  Zool. J. Linnean Society 172:501-520.

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.