Thursday, January 15, 2015

Genetic Distinction of two closely related unionid bivalve monotypic genera: Arcidens and Arkansia

Arcidens confragosus (INHS pic)
Discovering, describing and classifying the world’s species on the basis of evolutionary relationships is a laudable objective.   This is particularly important for taxa that are imperiled.  Unionid bivalves are recognized as one of the most imperiled groups of animals around the world with 70% of the recognized species in North America considered either extinct, endangered, threatened or of special concern (Williams et al., 1993; Neves et al., 1997; Lydeard et al., 2004).  Unionid species and genera have traditionally been diagnosed on characteristics of the shell and soft-parts.  One problem with relying on shell morphology is the fact that some variation has been attributed to environmental factors making it easier to over- or under-estimate species diversity depending on the circumstances. 

Arkansia wheeleri (INHS pic)
     Recently, an interesting paper entitled “Molecular phylogenetics and morphological variation reveal recent speciation in freshwater mussels of the genera Arcidens and Arkansia (Bivalvia: Unionidae)" was published in 2014 in the Biological Journal of the Linnean Society by Kentaro Inoue, Alyssa L. McQueen, John L Harris, and David J. Berg.    The authors examined the evolutionary history and taxonomic status of the two monotypic genera, which some have treated as belonging to separate genera (Turgeon et al., 1998) and others as congeners (e.g., Graf and Cummings, 2007). Phylogenetic analysis of sequences mitochondrial cytochrome oxidase I and NADH dehydrogenase subunit 1 and the nuclear internal transcribed spacer 1 revealed that the two species are reciprocally monophyletic, but very similar genetically.  Indeed, the estimate of divergence time based on mitochondrial data places their speciation event in the Holocene about 5,860 years ago when the streams of the Interior Highlands, where the federally endangered Arkansia wheeleri occurs, was isolated from the more widely distributed sister-species Arcidens confragosus.  Interestingly, traditional morphometric analysis showed distinct shell shapes between the species, but the geometric morphometrics did not thereby indicating that there is still value to conducting traditional morphometric studies of unionid shells to delineate species.  However, shell-shape variation should probably be examined in conjunction with a molecular phylogenetic analysis.  The authors propose that the two species be placed in the nomen Arcidens, which has priority.   

Literature Cited

Graf, D. L., K. S. Cummings.  2007.  Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida).  Journal of Molluscan Studies 73:291-314.

Lydeard, C., R. H. Cowie, W. F. Ponder, A. E. Bogan, P. Bouchet, S. A. Clark, K. S. Cummings, T. J. Frest, O. Gargominy, D. G. Herbert, R. Hershler, K. E. Perez, B. Roth, M. Seddon, E. E. Strong, and F. G. Thompson.  2004.  The global decline of nonmarine mollusks.  BioScience 54:321-330.

Neves, R. J., A. E. Bogan, J. D. Williams, S. A. Ahlstedt, and P. W. Haartfield.  1997.  Status of aquatic mollusks in the southeastern United States: a downward spiral of diversity.  In: G. W. Benz and D. E. Collins (eds.) Aquatic Fauna in Peril: the Southeastern Perspective.  Special Publication 1, Southeast Aquatic Research Institute, Lenz Design & Communications, Decatur, GA, 43-85. 

Turgeon, D. D., J. F. Quinn, Jr., A. E. Bogan, E. V. Coan, F. G. Hochberg, Jr., W. G. Lyons, P. M. Mikkelsen, R. J. Neves, C. F. E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F. G. Thompson, M. Vecchione, and J. D. Williams.  1998.  Common and scientific names of aquatic invertebrates from the United States and Canada: mollusks.  Speical Publication 26.  Bethesda, MD: American Fisheries Society.

Williams, J. D., M. L. Warren, Jr., K. S. Cumming, J. L. Harris, and R. J. Neves.  1993.  Conservation status of freshwater mussels of the United States and Canada.  Fisheries 18:6-22.

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.