Funding provided in part by NSF IOS 0743284
and
Laboratory of Molecular and Evolutionary Ecology
Dr. Nicole Valenzuela
Department of Ecology, Evolution, and Organismal BiologyIowa State University
239 Bessey Hall
Ames, IA 50011
(515) 294-1285
nvalenzu@iastate.edu
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Research Foci
The study of sex determination has important implications for our understanding of multiple traits and phenomena related to sexual reproduction, such as Sex Allocation and Sex Ratio Evolution,. Sexual Dimorphism and Sex-linked Traits.
Developmental Pathways
Phenotypic PlasticityMost of evolutionary biology relies on the assumption that genetic variation (i.e. differences in genomic composition) underlies the diversity of the phenotypes that are exposed to natural selection and allows its evolution. But a great proportion of the phenotypic variation we observe in nature derives from environmental sensitivity of the genome which influences its expression during development (regulatory differences). Discrete traits such as alternative morphs (e.g. threshold traits) may result from environmental modulation of the expression of major genes, rather than from the added expression of quantitative genes. TSD represents a form of phenotypic plasticity (a thermal sexual polyphenism) where identical genomes can permanently differentiate into either sex depending on the environmental conditions. We are interested in identifying how many distinct systems of sexual polyphenisms exist in nature, how do they work, and why did they come into being.
Is “TSD” a single or multiple mechanisms?One of the current research projects in the lab focuses on the evolution of the gene regulatory networks underlying sex determination by examining gene expression patterns across closely related TSD and GSD vertebrates (turtles). This candidate gene approach is shedding light on the fundamental question of whether TSD mechanisms evolved in one developmentally-conserved way, or whether multiple molecular pathways have evolved to produce ecologically equivalent outcomes. Our data on six critical sex differentiation genes (Aromatase, Sf1, Wt1, Dmrt1, Sox9, and Dax1) in painted turtles (Chrysemys picta: TSD) and softshell turtles (Apalone mutica: GSD) indicate that substantial evolutionary changes have accrued not only between TSD and GSD systems, but across TSD species as well (Valenzuela et al. 2006; Valenzuela and Shikano 2007). We have also shown that the fundamental difference between TSD and GSD mechanisms is not Aromatase-driven as previously proposed (Valenzuela and Shikano 2007). What renders TSD thermosensitive?Our comparative data have also addressed a crucial unanswered question of what molecular factor(s) renders TSD mechanisms thermosensitive. Potential candidates would be genes that express differentially by temperature prior to the onset of the thermosensitive period or TSP (genes organizing or activating the thermosensitive time window rather than those genes acting once the window has opened). By profiling gene expression early in development we have detected such differential expression in TSD turtles in two early-acting genes (Sf1 and Wt1) involved in the formation of the bipotential gonad (prior to the gonadal commitment to the ovarian or testicular differentiation pathways) (Valenzuela et al. 2006, Valenzuela 2008). Because Wt1 is a known regulator of Sf1, Wt1 is proposed as a more likely candidate TSD master switch. Is gene expression totally thermoinsensitive in GSD taxa?An additional evolutionary question that this comparative approach helps elucidate is whether GSD species derived from TSD ancestors have lost all thermal sensitivity in the regulation of the gene network underlying sexual differentiation. Data from A. mutica indicate that this is not always the case, as this GSD turtle has retained its ancestral sensitivity in the expression of a gene involved in gonadogenesis, the first such case ever to be reported (Valenzuela 2008). This result has paramount implications as it reveals that GSD taxa can harbor thermal sensitivity even when it is non-functional for sexual differentiation (temperature does not bias sex ratios in A. mutica). This is a critical finding because theoretical models for the evolution of TSD rely on the premise that GSD taxa posses an ubiquitous thermal sensitivity that can be co-opted during the evolution of phenotypic plasticity (TSD), and our data provide the first empirical evidence for its existence at the level of gene expression.
An ecological genomic project in my lab is aimed at elucidating the effect that fluctuating temperature has on the expression of genes involved in gonadogenesis in TSD turtles. This approach is essential because most molecular studies of TSD have been carried out at constant temperatures, yet sex ratios produced under constant conditions often differ from sex ratios obtained in the field where temperature fluctuates daily (e.g. Valenzuela et al. 1997, Valenzuela 2001a). Thus, the relationship between fluctuating temperatures in natural nests and offspring sex ratio remains obscure.
Sex Chromosome Evolution
Comparative Gene Mapping
Collaborators Links: Prof. Jenny A.M. Graves, Australian National University Prof. Arthur Georges, University of Canberra Prof. Russell Burke, Hofstra University Prof. Scott Edwards, Harvard University
Because we are interested in the comparative evolutionary genomics in an ecologically-relevant context, some of the research in my lab investigates questions in population and ecological genetics, life history evolution and conservation biology following previous research (Lance et al. 1992, Valenzuela et al. 1997, Valenzuela 2000, Valenzuela 2001a,b,c, Valenzuela & Janzen 2001, Morjan & Valenzuela 2001, Valenzuela et al. 2003, 2004, Pearse et al. 2006). This component, which addresses basic questions in evolutionary ecology, provides a critical view of the ecological framework in which sex determining mechanisms evolve and their evolutionary potential in the face of climate change.
Podocnemis unifilis
We are conducting a metapopulation genetic study of an endangered freshwater turtle (Podocnemis unifilis) inhabiting the Amazon and Orinoco river basins. This study parallels a previous collaboration done on a sister taxon (P. expansa) (Pearse et al. 2006).
Podocnemis expansa
We are also investigating the plasticity of body growth in P. expansa, a species we have studied intensely from a sex determination and ecological genetics perspective (Valenzuela 2000, Valenzuela 2001a,b,c).
Life History Evolution
Herpetology
Tropical Biology
Conservation Many
TSD reptiles are endangered. An important contribution to conservation
is providing managers with essential biological information to design,
evaluate, and enhance effective management programs. Part of our research
is devoted to addressing basic questions that have conservation implications,
such as the assessment of population structure, migratory and mating behavior
with the use of molecular markers. The understanding of how TSD works
in nature and under semi-natural or laboratory conditions is also important
for conservation practices.
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It
is fascinating that sexually-reproducing organisms employ such diversity
of mechanisms to produce males and females, ranging from systems under
strict genetic control (GSD) [such as highly dimorphic or undifferentiated
sex chromosomes (XY, ZW)], to genetic systems susceptible to some environmental
influences [such as haplo-dyploidy, polygenic systems, socially-induced
sex reversals], to systems under strict environmental control dependent
on biotic or abiotic factors. Among vertebrates, an environmental system
dependent on temperature (TSD) is commonly found in reptiles and fish.
Research
in my lab addresses the question of whether multiple molecular pathways
have evolved to produce ecologically equivalent TSD systems among taxa.
We use a comparative evolutionary genomic framework to study the gene
network underlying TSD and GSD in multiple species to identify the regulatory
gene(s) that render TSD systems thermo-sensitive. We are particularly
interested in understanding how TSD works in nature, and the relative
role of adaptive versus neutral processes during its evolution. 
Another
research avenue is the study the co-evolution between chromosomes and
sex-determining mechanisms. During a recent collaboration using comparative
genome hybridization, we discovered the existence of a cryptic XY sex
chromosome system in a GSD turtle from Australia (Chelodina longicollis),
which involved a pair of micro-chromosomes (Ezaz et al. 2006). This was
the first such report for turtles.
A
follow up study in a closely related species (Emydura maquarii)
revealed a similar cryptic XY system of macro-chromosomes (Martinez et
al. 2008). 
Through
an additional collaboration, we have started a comparative mapping study
genes involved in gonadogenesis across North American and Australian TSD
and GSD turtles. So far we have detected a conservative mapping for some
of these genes and other genes await future examination. 
Another important component of our research is the characterization of
reptilian life histories, theoretical and empirical TSD thermal ecology,
mating systems, population genetics, and reproductive behavior. In particular,
we study South American river turtles of the genus Podocnemis,
the North American painted turtle (Chrysemys picta), and snapping
turtle (Chelydra serpentina). 
Reptiles
are a very good taxon for study because TSD and GSD co-occur in this group.
We use turtles as a model system to study the evolution of sex determining
mechanisms, and compare both TSD and GSD species from the tropics and
the temperate zone.