title: Historical Biogeography of Lowland Species of Toads (Bufo) Across the Trans-Mexican Neovolcanic Belt and the Isthmus of Tehuantepec
Creators: name: Morrill, Benson H.
Publisher: name: Utah State University
Year: 2004

description: The eastern extension of the massive Trans-Mexican Neovolcanic Belt 1 (TMNB) contacts the Atlantic Coast in central Veracruz, Mexico. Although it is not a massive structure at this eastern terminus, the TMNB has nonetheless effected vicariance and subsequent speciation in several species groups, including bufonid toads and freshwater fishes. In this study, we examined mtDNA sequences (cyt b, 16S) from populations of two different species groups of toads (B. marinus, and the B. nebulifer-B. valliceps species pair) from a lowland transect across the eastern end of the TMNB. We also included samples from outside this region (e.g., Costa Rica, Honduras, northern Mexico) for both species groups. Our results further define the parapatric contact zone between the Pliocene-vicariant, sister-species B. nebulifer and B. valliceps. In contrast, we found no significant phylogenetic structure among populations of B. marinus across either side of the TMNB, suggesting a more recent Pleistocene dispersal in this group. In addition, we found phylogenetic structure associated with the Isthmus of Tehuantepec in both species groups. The Isthmus of Tehuantepec unites the North American continent with nuclear Central America, and the presence of a Pliocene seaway across this region has been controversial. Our data support clades on either side of the isthmus within two relatively unrelated species (B. valliceps and B. marinus). Both of these species contain clades that are consistent with a late Pliocene divergence, which supports the hypothesis of a marine barrier across the Isthmus of Tehuantepec at this time. The divergence between B. valliceps and B. nebulifer across the TMNB is further supported, and appears to predate the isthmian break in this complex, whereas the B. marinus group shows a dispersal across the TMNB subsequent to the presence of a late-Pliocene seaway over the Isthmus of Tehuantepec.
The Trans-Mexico Neovolcanic Belt (TMNB) is one of the predominant geographical features of Mexico, and its geological development has been posited as a primary contributor to the biogeographic histories of many upland taxa in central Mexico (e.g., Campbell and Frost, 1993; Darda, 1994; Parkinson et al., 2000; Sullivan et al., 2000; Castoe et al., 2003). However, a recent series of papers (Mulcahy and Mendelson, 2000; Hulsey et al., 2004; Zaldivar-Riverón et al, 2004) have independently demonstrated the considerable influence of this transverse massif on the biogeography and evolution of the lowland fauna along the central Atlantic Versant of Mexico. These papers generally support an earlier hypothesis implied by Perez-Higareda and Navarro (1980): based on their observation of consistent disjunctions in the distribution of subspecies (reptiles and mammals) along the Atlantic Coast, they suggested that the TMNB may form a geographical barrier to lowland species in the region.
The concept of a massive volcanic chain such as the TMNB acting as a vicariant feature to lowland populations is easily tractable, but the reality is that the TMNB withers to a tiny string of lava-rock strewn hills at its eastern terminus. It makes final contact with the current coastline, as two small fingers of raised lava-rock, just south of the town of Palma Sola, Veracruz, Mexico. Unlike the imposing backbone of the TMNB, which boasts the highest peaks of Mexico such as Pico de Orizaba and Cofre de Perote, the eastern fingers of the TMNB are barely noticeable to the casual traveler in the region. Mulcahy and Mendelson (2000) recounted the history of seawater inundations in this region that included Miocene-Pliocene-era maxima that effectively covered the entire coastal plain of central Mexico and Pleistocene-era minima that exposed large sections of the continental shelf to greatly increase the areal extent of the coastal plain. By comparing phylogenetic signal from mtDNA sequences of lowland toads in this region, Mulcahy and Mendelson (2000) demonstrated that the prevailing concept of a wide-ranging single species (Bufo valliceps Wiegmann) was actually a species pair showing an apparent parapatric distribution in the region: B. valliceps ranging from central Veracruz, Mexico, southward to Costa Rica; and B. nebulifer Girard, ranging from central Veracruz northward to the southern United States. Mulcahy and Mendelson (2000) proposed and tested two historical hypotheses related to the timing of this speciation event: 1) Miocene-Pliocene vicariance associated with the orogeny of the TMNB; and 2) Pleistocene vicariance and dispersal associated with raised sea levels that obliterated the coastal plain in this region; their results supported the Miocene-Pliocene vicariance hypothesis. However, their sampling was not sufficient enough to allow them to demonstrate strict parapatry on the narrow coastal plains on either side of the TMNB. In this paper we use recently collected samples from a geographic transect in the central part of coastal Veracruz, Mexico, to test the hypothesis of parapatry of B. nebulifer and B. valliceps across the eastern terminus of the TMNB (Mulcahy and Mendelson, 2000). We also propose and test a related hypothesis that the sympatric toad B. marinus shows a more recent dispersal across the TMNB, consistent with the Pleistocene vicariance and dispersal hypothesis of Mulcahy and Mendelson (2000). We test this hypothesis based on the fact that B. marinus is from South American stock, and most likely entered lower Central America during the Pliocene. A broad scale phylogeographic study of B. marinus (Slade and Moritz, 1998) indicated that this complex showed dramatic historical effects of the orogeny of the Andes in South America, and some level of genetic divergence between samples from Costa Rica and Mexico. These results suggest that, in our finer scale study, the biogeographic history of northern B. marinus may show an influence of the TMNB similar to that already documented in other toads in the region (i.e., B. valliceps and B. nebulifer).
The three species of toads in our study are ecologically similar in their general reproductive biology and overall natural history. All three species are invasive, "weedy" species that are typically more abundant in secondary, degraded habitats than in undisturbed primary forests (Mendelson, 1994). These attributes would suggest that they are suitably comparable to one another, in order to test hypotheses of historical biogeography, and also that their invasive, dispersalist tendencies would make them a conservative test of any potential vicariant effect of the TMNB on lowland species.
In this project we use our study system to address three principal questions regarding the historical biogeography of the Atlantic versant lowlands of Mexico: 1) the parapatry of B. valliceps and B. nebulifer at the the eastern terminus of the TMNB; 2) evidence of phylogenetic breaks within B. valliceps and/or B. marinus that are consistent with hypothesized seaways across the Isthmus of Tehuantepec; 3) evidence of Pleistocene dispersal of B. marinus across the TMNB.