Rachel Lockridge Mueller
Assistant Professor
Department of Biology
Colorado State University
Fort Collins, CO 80523
rlm@colostate.edu
(970) 491-6717
|
P l e t h o d o n t i d S a l a m a n d e r S y s t e m a t i c s
Life History Evolution and Morphological Homoplasy—More than two-thirds of all salamanders are members of Plethodontidae, the lungless salamanders. Many lineages within this clade show long-term morphological stasis, while others have evolved diverse life history strategies, ecologies, and morphological specializations. Thorough analysis of plethodontid evolutionary history has been precluded by the lack of a molecular phylogenetic hypothesis. Using 27 complete mitochondrial genome sequences, I estimated a phylogeny for plethodontid and related salamanders that differs markedly from morphological phylogenetic hypotheses (Mueller et al. Proc. Nat. Acad. Sci., 2004). Using this topology and fossil calibrations, I also estimated much earlier divergence dates for major groups of salamanders than were previously reported. These new phylogenetic relationships suggest re-evolution of a biphasic life history, including a larval stage, from at least one direct-developing ancestral species; this evolutionary transformation was previously considered unlikely. Additionally, these results demonstrate that the evolution of many plethodontid traits is characterized by extensive parallel evolution and reversal to an unspecialized ancestral form. Such traits include: (1) tails specialized for autotomy, (2) reduction in number of toes, (3) highly modified projectile tongues, and (4) enucleated red blood cells.
Red Blood Cell Morphology and the Evolution of Genome Size—Plethodontid salamanders have a large range of nuclear genome sizes—14 to 70 billion nucleotides per haploid nucleus—and are therefore an excellent system with which to study whole-genome evolution and its morphological consequences. Five plethodontid clades, including the genus Batrachoseps, have independently evolved enucleated red blood cells. This cell morphology is extremely unusual among non-mammalian vertebrates. All plethodontids with such cells have large genomes and miniaturized body forms, two features that interact to effect morphological simplification in the nervous, visual, and skeletal systems. I have used cell morphological data, genome size data, and a molecular phylogeny for Batrachoseps to demonstrate that miniaturization and large genomes are also correlated with highly variable red blood cell morphology (Mueller et al., Zoology, in press). I plan to use simulations to test which model of genome size evolution most likely yielded the variable distribution of genome sizes among plethodontids.
|
