Mary E. White

Evolution of predetermined germ cells in vertebrate embryos: implications for macroevolution

Summary The germ line is established in animal embryos with the formation of primordial germ cells (PGCs), which give rise to gametes. Therefore, the need to form PGCs can act as a developmental constraint by inhibiting the evolution of embryonic patterning mechanisms that compromise their development. Conversely, events that stabilize the PGCs may liberate these constraints. Two modes of germ cell determination exist in animal embryos: (a) either PGCs are predetermined by the inheritance of germ cell determinants (germ plasm) or (b) PGCs are formed by inducing signals secreted by embryonic tissues (i.e., regulative determination). Surprisingly, among the major extant amphibian lineages, one mechanism is found in urodeles and the other in anurans. In anuran amphibians PGCs are predetermined by germ plasm; in urodele amphibians PGCs are formed by inducing signals. To determine which mechanism is ancestral to the tetrapod lineage and to understand the pattern of inheritance in higher vertebrates, we used a phylogenetic approach to analyze basic morphological processes in both groups and correlated these with mechanisms of germ cell determination. Our results indicate that regulative germ cell determination is a property of embryos retaining ancestral embryological processes, whereas predetermined germ cells are found in embryos with derived morphological traits. These correlations suggest that regulative germ cell formation is an important developmental constraint in vertebrate embryos, acting before the highly conserved pharyngula stage. Moreover, our analysis suggests that germ plasm has evolved independently in several lineages of vertebrate embryos.

Palm‐Pitviper (Bothriechis) Phylogeny, mtDNA, and Consilience

The phylogeny of the neotropical palm‐pitviper genus Bothriechis has been previously inferred from morphology and allozymes. These nuclear‐based data sets were found to be congruent and also consilient with the geologic history of the region. We present mtDNA sequence data as an additional data set in the inference of Bothriechis phylogeny and analyze it separately and combined with previous data. The mtDNA phylogeny is incongruent with the nuclear data sets. Based on a number of factors, we hypothesize that the incongruence is due to both mtDNA introgression and lineage sorting. We argue that mtDNA represents extrinsic data and as such should be used as a consilient data set.

Gene conversions may obscure actin gene family relationships.

Abstract. Phylogenetic hypotheses of muscle actin evolution are significantly different when a sea urchin is used as a representative echinoderm than when a sea star is used. While sea urchin muscle actins support an echinoderm–chordate sister relationship, sea star sequences suggest that echinoderm muscle actins are convergent with chordate muscle actins. Our results suggest that gene conversion in the sea star muscle actin may be responsible for these discordant results.

A Reevaluation of the Status of the Foxsnakes Pantherophis gloydi Conant and P. vulpinus Baird and Girard (Lepidosauria)

As currently understood, there are two species of foxsnakes (Eastern Foxsnake, Pantherophis gloydi Conant and Western Foxsnake, P. vulpinus Baird and Girard) that are separated by a large geographic disjunction that encompasses almost all of Michigan, eastern Indiana, and eastern Ohio. Phylogenetic analysis of mtDNA data of individuals from throughout the ranges of the two species inferred reciprocally monophyletic clades that revealed a new species boundary, the Mississippi River. The single key morphological character also shows a major difference at the river. Because the localities of the holotypes of P. gloydi and P. vulpinus are both within the new range of the eastern form, gloydi is recognized as a junior synonym of vulpinus and a new name, P. ramspotti, is erected for the western form. The estimates of divergence time and historical biogeography suggest that Pleistocene glaciation and the Mississippi River played a key role in speciation.

Crocodylian nuclear factor kappa B

We deduced the amino acid (aa) sequence of the nuclear factor kappa B (NFκB) protein from genomic data for the American alligator (Alligator mississippiensis), the estuarine crocodile (Crocodylus porosus), and the Indian gharial (Gavialis gangeticus). A 105kDa protein, NFκB1 exhibits complex post-translational processing, multiple mechanisms of activation, and acts as precursor for a p50, a Rel homology transcription factor which influences the expression of key genes for developmental processes, apoptosis, and immune function. The aa sequences of the crocodylian proteins share very high identity with each other (97.2±0.7%), birds (81.0±1.1%, n=6), mammals (75.3±1.6%, n=4), reptiles (80.3±5.1%, n=2), and less identity with fish (55.5±5.5%, n=4) and one amphibian (66.1±0.8%). The crocodylian protein has a well-conserved Rel homology domain, a nuclear localization signal, and a glycine-rich region which facilitates proteasome-mediated generation of p50. The Rel homology domain contains sequences responsible for dimerization, DNA-binding, and nuclear translocation. In addition, seven ankyrin repeats were located, which putatively allow for inhibition of transcriptional regulation by mediating interaction with Inhibitor kappa B. Other features include a death domain, and conserved serine residues, near the C-terminal end, which act as potential phosphorylation sites for activation of the proteolytic generation of p50. Western blot analysis showed both the 105kDa precursor and the 50kDa mature NFκB were expressed in the alligator liver. Nuclear factor κB exhibited diffuse cytoplasmic distribution in alligator hepatocytes, and almost no cytoplasmic localization in infected animals. In addition, nuclear NFκB exhibited specific binding to the consensus NFκB promoter element.

Two Different Complement C3 Genes in Crocodilians

Crocodilians exhibit powerful antibacterial activities in their tissues and blood. The activities have been partially attributed to the presence of a potent serum complement system of proteins that acts in a nonspecific manner to kill bacteria. Complement activation involves activation of complement C3, a component with broad immune and regulatory function. We searched the crocodilian genomes (Alligator mississippiensis, Crocodylus porosus, and Gavialis gangeticus) for complement C3, and found two genes that code for isoforms with quite different sequences. Birds and mammals express only a single isoform of the complement C3 protein. Some snakes were shown to have two C3 genes; however, these encode proteins with very similar amino acid sequences. To date, only fishes were reported to express complement C3 isoforms with diversity similar to that of the crocodilian forms. Phylogenetic analysis suggests the gene duplication leading to the two crocodilian paralogs probably occurred within the order Crocodylia. Both contain signal sequences, putative internal thioesters, potential N-glycosylation sites, and functional domains that would allow them to interact with complement receptors and other complement system components. As has been suggested with fishes, the expression of multiple functional C3 isoforms may allow crocodilians to respond to a broad spectrum of immunological insult.

Diversification and Germ-Line Determination Revisited: Linking Developmental Mechanism with Species Richness

Abstract.– Background: Explanations for asymmetric patterns of diversification continue to challenge paleontologists and neontologists with competing hypotheses within genetic-development and ecological frameworks. In 1988, a hypothesis was proposed that tied a primordial germ cell (PGC) determination mechanism to clade (phyla) diversification. Two general mechanisms for PGC determination are recognized: one is termed induced because induction signals are required for the production of primordial germ cells. The other mechanism is cell-autonomous, i.e. determinative, because the cells that develop in response to specific cytoplasmic determinants in the oocyte are pre-destined to become PGCs. We revisited the hypothesis and analyzed phyla diversity with germ cell determination mechanisms and examined sister clade asymmetry. Results: After 25 years of additional data accumulation, the hypothesis that high levels of species diversification are associated with the induced mode is falsified, with the determinative mode revealed as associated with higher rates of diversification. The greater species numbers are significantly associated (ANOVA p>0.003) with the determinative mode. Analysis with appropriate sister clades is unanimous in showing the clade with the determinative mode has a significantly greater number of species relative to its induced sister clade . Conclusions: The primordial germ cell determination mechanism hypothesis explains asymmetrical species diversity and morphological disparity at the phylum level. We argue that the determinative mode of primordial germ cell determination is a constraint release that has enhanced evolvability and increased rates of speciation and morphological disparity among clades. Knowledge of the mechanism for extant theropods allows speculation that its sister clade, the Sauropodomorpha would have exhibited the induced mode. Results: After 25 years of additional data accumulation, the hypothesis that high levels of species diversification are associated with the induced mode is falsified, with the determinative mode revealed as associated with higher rates of diversification. The greater species numbers are significantly associated (ANOVA p>0.003) with the determinative mode. Analysis with appropriate sister clades is unanimous in showing the clade with the determinative mode has a significantly greater number of species relative to its induced sister clade . Conclusions: The primordial germ cell determination mechanism hypothesis explains asymmetrical species diversity and morphological disparity at