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Tim Karr

Arizona State University
1001 S. McAllister Avenue
Tempe, AZ 85287
US_United States_USD


Taxa Studied: Invertebrate Animals, Vertebrate Animals
Techniques Employed: Quantitative PCR (qPCR), Microarrays, Solexa (Illumina) Sequencing, Bioinformatics/Sequence Analysis, In Situ Hybridization, Antibody Staining, Epifluoresence Microscopy, Confocal Microscopy, Transgenesis, RNA interference(RNAi)
Research Description: The origins of sexual reproduction remain a mystery. Likewise, the origins of both sperm and egg remain obscure in part due to inadequate understanding of their molecular composition and respective evolutionary histories. Sperm are unique, ‘simple’ cells as shown in recent publications (e.g., the Drosophila sperm proteome contains approxi-mately 1100 proteins, the mouse about 1000), and this manageable number represents a dataset amenable to analysis at the experimental, evolutionary, and theoretical model-ing levels. Some surprises have already surfaced- e.g., the Drosophila sperm proteome consists of (i) novel proteins encoded by lineage-specific and recently expanded gene families, (ii) numerous unnanotated proteins and (iii) an overabundance of retroposed genes. Thus, this relatively simple cell type may act as an incubator for novel protein evolution and function. These, and other findings concerning the fate of sperm following fertilization, provides a fresh new view of the evolution of sexual reproduction and has led to some novel ideas, still in their infancy, about the origins of sperm and the evolution of sex. Briefly, according to one scenario, sperm evolved as the result of an ancient “in-teraction” between two disparate cell types that resulted in the formation of an ancestral “Ur-spermatozoa”. This event occurred very early in the evolution of multi-cellularity, es-timated to be around the same time as the Cambrian diversification (i.e, ~600 Mya). The cellular novelty of the Ur-spermatozoon, and its impact on the genetic architecture of the emerging diploid cell was so great that it dominated evolutionary and fitness landscapes and ultimately led to a semi-stable state of “sex” as we know it today. Although this rep-resents but one of many ideas on the topic, none have focused per se on the origin of sperm, its success (fixation in the metazoa), nor its persistence throughout the Eu-karyota. One practical advantage of a ‘sperm-centric’ focus is our ability to rapidly produce sperm proteomes across diverse taxa and then use the information to define the “core sperm proteome”. Also, spermatozoa represent clearly homologous cell types throughout the animal kingdom and is present in all basal metazoans (ctenophores, cnidarians, sponges, and bilaterians). We hope to ultimately reconstruct the ancestral “Ur-spermatozoon” cell type model from which we can probe the evolutionary history of this seminal evolutionary event back to the root of multicellular life. The primary research approach centers around mass spectrometry of whole sperm, a proven approach for elucidating sperm proteomes. We are characterizing conserved and novel features of each taxon and relating them to their phylogenetic position and life history strategy to determine loci of strong stabilizing or sexual selection. These data will not only help elu-cidate how individual cell types evolved from a common progenitor early in the evolution of life and how molecules evolved in the context of a functioning cell type, but they might also identify conserved features of spermatozoon function (e.g. chromatin stabil-ity/packing, DNA binding proteins, etc) relevant to human reproduction.
Lab Web Page:
Willing to Host Undergraduates: YES
Actively Seeking Undergraduates: YES
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