Paul T. Chippindale

PATTERNS OF POSTZYGOTIC ISOLATION IN FROGS

From literature data on 116 taxa crosses involving 46 species of frogs, we found a positive correlation between degree of divergence (measured as Neis genetic distance, D) and degree of postzygotic isolation. In anurans, hybrid sterility appears to evolve more quickly than inviability, which is consistent with the conclusions of other studies that involved Drosophila species. The lower threshold of D = 0.30 for evolution of hybrid inviability that we found is similar to that observed for Drosophila. This consistency suggests that there may be a general pattern in the acquisition of reproductive isolation in animals.

PHYLOGENETIC EVIDENCE FOR A MAJOR REVERSAL OF LIFE-HISTORY EVOLUTION IN PLETHODONTID SALAMANDERS

Abstract The transition from aquatic to terrestrial eggs is a key evolutionary change that has allowed vertebrates to successfully colonize and exploit the land. Although most amphibians retain the primitive biphasic life cycle (eggs deposited in water that hatch into free‐living aquatic larvae), direct development of terrestrial eggs has evolved repeatedly and may have been critical to the evolutionary success of several amphibian groups. We provide the first conclusive evidence for evolutionary reversal of direct development in vertebrates. The family Plethodontidae (lungless salamanders) contains the majority of salamander species, including major radiations of direct developers. We reconstruct the higher level phylogenetic relationships of plethodontid salamanders using molecular and morphological data and use this phylogeny to examine the evolution of direct development. We show that the predominantly biphasic desmognathines, previously considered the sister group of other plethodontids, are nested inside a group of directdeveloping species (Plethodontini) and have re‐evolved the aquatic larval stage. Rather than being an evolutionary dead end, the reversal from direct developing to biphasic life history may have helped communities in eastern North America to achieve the highest local diversity of salamander species in the world.

When are phylogenetic analyses misled by convergence? A case study in Texas cave salamanders.

Convergence, i.e., similarity between organisms that is not the direct result of shared phylogenetic history (and that may instead result from independent adaptations to similar environments), is a fundamental issue that lies at the interface of systematics and evolutionary biology. Although convergence is often cited as an important problem in morphological phylogenetics, there have been few well-documented examples of strongly supported and misleading phylogenetic estimates that result from adaptive convergence in morphology. In this article, we propose criteria that can be used to infer whether or not a phylogenetic analysis has been misled by convergence. We then apply these criteria in a study of central Texas cave salamanders (genus Eurycea). Morphological characters (apparently related to cave-dwelling habitat use) support a clade uniting the species E. rathbuni and E. tridentifera, whereas mitochondrial DNA sequences and allozyme data show that these two species are not closely related. We suggest that a likely explanation for the paucity of examples of strongly misleading morphological convergence is that the conditions under which adaptive convergence is most likely to produce strongly misleading results are limited. Specifically, convergence is most likely to be problematic in groups (such as the central Texas Eurycea) in which most species are morphologically very similar and some of the species have invaded and adapted to a novel selective environment.

Vicariant Origin of Malagasy Reptiles Supports Late Cretaceous Antarctic Land Bridge

Since the acceptance of Wegener’s theory of plate tectonics in the 1960s, continental drift vicariance has been proposed as an explanation for pan‐Gondwanan faunal distributions. Given the recognition of historical connections among continents, it no longer was necessary to invoke hypotheses of dispersal across nearly insurmountable barriers. The application of continental drift vicariance theory to Gondwanan floral and faunal distributions provided reasonable explanations for such unusual distributions as that of the southern beech (Nothofagus) and chameleons. However, recent studies have demonstrated a significant, if not dominant, role for dispersal in the present‐day distributions of these and numerous other “Gondwanan” taxa. The evolutionary histories of three Malagasy groups (boid snakes, podocnemid turtles, and iguanid lizards) commonly have been interpreted as reflecting vicariance because of continental drift associated with the breakup of Gondwana. Bayesian analyses of divergence ages suggest that this pattern is the result of vicariance coincident with the isolation of Madagascar in the Late Cretaceous (∼80 million years ago). This represents the first temporal evidence linking the vicariant origin of extant Malagasy vertebrates to a single geologic event. Specifically, our data provide strong, independently corroborated evidence for a contiguous Late Cretaceous Gondwana, exclusive of Africa and connected via Antarctica.

Phylogeny of Frogs of the Physalaemus Pustulosus Species Group, With an Examination of Data Incongruence

Characters derived from advertisement calls, morphology, allozymes, and the sequences of the small subunit of the mitochondrial ribosomal gene (12S) and the cytochrome oxidase I (COI) mitochondrial gene were used to estimate the phylogeny of frogs of the Physalaemus pustulosus group (Leptodactylidae). The combinability of these data partitions was assessed in several ways: measures of phylogenetic signal, character support for trees, congruence of tree topologies, compatibility of data partitions with suboptimal trees, and homogeneity of data partitions. Combined parsimony analysis of all data equally weighted yielded the same tree as the 12S partition analyzed under parsimony and maximum likelihood. The COI, allozyme, and morphology partitions were generally congruent and compatible with the tree derived from combined data. The call data were significantly different from all other partitions, whether considered in terms of tree topology alone, partition homogeneity, or compatibility of data with trees derived from other partitions. The lack of effect of the call data on the topology of the combined tree is probably due to the small number of call characters. The general incongruence of the call data with other data partitions is consistent with the idea that the advertisement calls of this group of frogs are under strong sexual selection.

Rapid diversification, incomplete isolation, and the "speciation clock" in North American salamanders (Genus Plethodon): testing the hybrid swarm hypothesis of rapid radiation.

Abstract The history of life has been marked by several spectacular radiations, in which many lineages arise over a short period of time. A possible consequence of such rapid splitting in the recent past is that the intrinsic barriers that prevent gene flow between many species may have too little time to develop fully, leading to extensive hybridization among recently evolved lineages. The salamander genus Plethodon in eastern North America has been proposed as a possible example of this scenario, but without explicit statistical tests. In this paper, we present a nearly comprehensive phylogeny for the 45 extant species of eastern Plethodon, based on DNA sequences of mitochondrial (two genes, 1335 base pairs) and nuclear genes (two genes, up to 3481 base pairs). We then use this phylogeny to examine rates and patterns of diversification and hybridization. We find significantly rapid diversification within the glutinosus species group. Examining patterns of natural hybridization in light of the phylogeny shows considerable hybridization within this clade, including introgression between species that are morphologically distinct and distantly related. Reproductive isolation increases over time and may be very weak among the most recently diverged species. These results suggest that the origin of species and the evolution of intrinsic reproductive isolating mechanisms, rather than being synonymous, may be decoupled in some cases (i.e., rapid origin of lineages outstrips the “speciation clock”). In contrast to the conclusions of a recent review of adaptive radiation and hybridization, we suggest that extensive hybridization sometimes may be a consequence, rather than a cause, of rapid diversification.

PHYLOGENETIC RELATIONSHIPS AND SYSTEMATIC REVISION OF CENTRAL TEXAS HEMIDACTYLIINE PLETHODONTID SALAMANDERS

Genetic variation and phylogenetic relationships of central Texas nontransforming spring and cave salamanders, genera Eurycea and Typhlomolge (Plethodontidae: Plethodontinae: Hemidactyliini), were examined using 25 allozyme loci and DNA sequence data for a maximum of 356 bp of the mitochondrial cytochrome b gene. Monophyly of the central Texas hemidactyliines is well supported. High levels of divergence occur among many populations and groups of populations, and there clearly are many more species in the group than previously recognized. Many have extremely restricted distributions in isolated islands of aquatic habitat. Several major monophyletic groups were identified that correspond to geographically circumscribed areas of the Edwards Plateau region. The deepest phylogenetic split in the group occurs between populations northeast versus southwest of the Colorado River. Species that have been assigned to the genus Typhlomolge are phylogenetically nested within the central Texas Eurycea; therefore, the genus Typhlomolge is placed in the synonymy of Eurycea. Continued recognition of the species E. latitans, E. nana, E. neotenes, E. pterophila, E. sosorum, E. tridentifera, and E. troglodytes is recommended, but E. neotenes appears to be restricted in range to a small geographic area, and is not widespread in the region as previously thought. The E. latitans and E. troglodytes species complexes are recognized; each con- sists of spring and cave populations that include those at the type localities of the latter two species, plus other populations to which they appear most closely related. Three new species from northeast of the Colorado River are described.

Speciation, phylogeography and evolution of life history and morphology in plethodontid salamanders of the Eurycea multiplicata complex.

Understanding the complex interactions among environment, genotype and ontogeny in determining organismal phenotypes is cental to many biological disciplines. The Eurycea multiplicata complex, endemic to the Interior Highlands (Ozark Plateau and Ouachita Mountains) of eastern North America, comprises a diverse radiation of paedomorphic surface‐dwelling (E. tynerensis), metamorphic surface‐dwelling (E. multiplicata multiplicata and E. m. griseogaster) and metamorphic subterranean (Typhlotriton spelaeus) hemidactyliine plethodontid salamanders. Portions of two mitochondrial genes, cytochrome‐b and NADH dehydrogenase‐4, totalling 1818 base pairs (bp) were sequenced for 70 ingroup individuals plus numerous outgroup taxa, to examine the biogeography and relationships among these morphologically disparate species. Results show the E. multiplicata complex to be monophyletic, with its two most divergent clades corresponding to geography, not morphology or life history. Transforming surface‐dwelling populations from the Ouachitas (E. m. multiplicata) are sister to the Ozark taxa, including paedomorphic surface‐dwelling (E. tynerensis), subterranean (T. spelaeus) and transforming surface‐dwelling salamanders assigned to the ‘subspecies’E. m. griseogaster. Among Ozark taxa T. spelaeus (deeply nested within Eurycea) is sister to a clade that includes E. m. griseogaster and E. tynerensis. Current taxonomy suggests that paedomorphic populations (E. tynerensis) from the western Ozarks are distinct from nearby transforming populations (E. m. griseogaster). However, paedomorphic and transforming salamanders do not form reciprocally monophyletic groups and many populations share almost identical haplotypes. Ancestral state reconstruction of life history traits shows that paedomorphosis arose independently from three to nine times. Most populations are either completely paedomorphic or completely transforming. This suggests that local habitat parameters strongly influence life history mode in this complex, either facultatively or by selection for particular genotypes.

Evolution of the Schlafen genes, a gene family associated with embryonic lethality, meiotic drive, immune processes and orthopoxvirus virulence

Genes of the Schlafen family, first discovered in mouse, are expressed in hematopoietic cells and are involved in immune processes. Previous results showed that they are candidate genes for two major phenomena: meiotic drive and embryonic lethality (DDK syndrome). However, these genes remain poorly understood, mostly due to the limitations imposed by their similarity, close location and the potential functional redundancy of the gene family members. Here we use genomic and phylogenetic studies to investigate the evolution and role of this family of genes. Our results show that the Schlafen family is widely distributed in mammals, where we recognize four major clades that experienced lineage-specific expansions or contractions in various orders, including primates and rodents. In addition, we identified members of the Schlafen family in Chondrichthyes and Amphibia, indicating an ancient origin of these genes. We find evidence that positive selection has acted on many Schlafen genes. Moreover, our analyses indicate that a member of the Schlafen family was horizontally transferred from murine rodents to orthopoxviruses, where it is hypothesized to play a role in allowing the virus to survive host immune defense mechanisms. The functional relevance of the viral Schlafen sequences is further underscored by our finding that they are evolving under purifying selection. This is of particular importance, since orthopoxviruses infect mammals and include variola, the causative agent of smallpox, and monkeypox, an emerging virus of great concern for human health.

SYSTEMATICS OF PYTHONS OF THE MORELIA AMETHISTINA COMPLEX (SERPENTES: BOIDAE) WITH THE DESCRIPTION OF THREE NEW SPECIES

The scrub pythons (Morelia amethistina complex) are revised based on museum specimens and new material recently collected in eastern Indonesia. Morelia kinghorni (formerly M. amethistina kinghorni) and M. amethistina (formerly M. amethistina amethistina) are recognized as species, and three new species are described. The phylogenetic relationships of scrub pythons are resolved using morphological and molecular characters. Scrub pythons are most closely related to Morelia boeleni and have undergone both ancient divergences and a relatively recent radiation. The distribution of scrub pythons corresponds well with areas of endemism recognized in earlier studies of other taxa. Their distribution and evolution appears to have been shaped by combined effects of dispersal and vicariance. Scrub python populations exhibit interesting color and pattern polymorphism and ontogenetic change, and these characteristics vary among populations.