Pattern does not equal process: exactly when is sex environmentally determined?
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Biomedical subjects
Fredric J Janzen
Publications and source records attributed to Fredric J Janzen.
Molecular systematics, phylogeography, and the effects of Pleistocene glaciation in the painted turtle (Chrysemys picta) complex.
The painted turtle, Chrysemys picta, is currently recognized as a continentally distributed polytypic species, ranging across North America from southern Canada to extreme northern Mexico. We analyzed variation in the rapidly evolving mitochondrial control region (CR) in 241 turtles from 117 localities across this range to examine whether the painted turtle represents a continentally distributed species based on molecular analysis. We found strong support for the novel hypothesis that C. p. dorsalis is the sister group to all remaining Chrysemys, with the remaining Chrysemys falling into a single, extremely wide-ranging and genetically undifferentiated species. Given our goal of an evolutionarily accurate taxonomy, we propose that two evolutionary lineages be recognized as species within Chrysemys: C. dorsalis (Agassiz 1857) in the southern Mississippi drainage region, and C. picta (Schneider 1783) from the rest of the range of the genus. Neither molecular nor recent morphological analyses argue for the hybrid origin of C. p. marginata as previously proposed. Within C. picta, we find evidence of at least two independent range expansions into previously glaciated regions of North America, one into New England and the other into the upper Midwest. We further find evidence of a massive extinction/recolonization event across the Great Plains/Rocky Mountain region encompassing over half the continental United States. The timing and extent of this colonization is consistent with a recently proposed regional aridification as the Laurentide ice sheets receded approximately 14,000 years ago, and we tentatively propose this paleoclimatological event as a major factor shaping genetic variation in Chrysemys.
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Phenotypic variation in smooth softshell turtles (Apalone mutica) from eggs incubated in constant versus fluctuating temperatures.
Temperatures experienced during embryonic development elicit well-documented phenotypic variation in embryonic and neonatal animals. Most research, however, has only considered the effects of constant temperatures, even though developmental temperatures in natural settings fluctuate considerably on a daily and seasonal basis. A laboratory study of 15 clutches of smooth softshell turtles (Apalone mutica) was conducted to explicitly examine the influence of thermal variance on phenotypic variation. Holding mean temperature constant and eliminating substrate moisture effects permitted a clear assessment of the impact of thermal variance on hatching success, incubation length, hatchling body size, swimming speed, and righting time. Incubation length and swimming speed varied significantly among temperature treatments. Both traits tended to increase with increasing thermal variance during embryonic development. Clutch significantly affected all traits examined, except righting time, even after accounting for the effects of initial egg mass. These results highlight the importance of accounting for the impact of both thermal mean and variance on phenotypic variation. The findings also strengthen the increasing recognition of maternal clutch effects as critical factors influencing phenotypic variation in neonatal animals.
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Molecular phylogeography of common garter snakes (Thamnophis sirtalis) in western North America: implications for regional historical forces.
Complete ND2 and partial ND4 and cytochrome b mitochondrial DNA (mtDNA) sequences were analysed to evaluate the phylogeographic patterns of common garter snakes (Thamnophis sirtalis) in western North America. This species is widely distributed throughout North America, and exhibits extensive phenotypic variation in the westernmost part of its range. The overall phylogeographic pattern based on mtDNA sequences is concordant with results from studies of other species in this region, implicating historical vicariant processes during the Pleistocene and indicating bottleneck effects of recent dispersal into postglacial habitat. Indeed, the topology is statistically consistent with the hypothesis of both southern (Great Basin and California) and northern (Haida Gwaii) refugia. Specifically, we identified genetic breaks among three major clades: Northwest Coastal populations, Intermountain populations, and all California populations. The California clade contained the only other well-supported branching patterns detected; relationships among populations within the two northern clades were indistinguishable. These molecular splits contrast sharply with all prior geographical analyses of phenotypic variation in T. sirtalis in this region. Our results suggest that the extensive phenotypic variation in western T. sirtalis has been shaped more by local evolutionary forces than by shared common ancestry. Consequently, we consider all morphologically based subspecies designations of T. sirtalis in this region invalid because they do not reflect reciprocal monophyly of the mtDNA sequences.
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