Takuya Nakazato
tnakazat@indiana.edu

Indiana University
Department of Biology
1001 E. Third Street
Bloomington, IN 47405
USA

Advisors:
Dr. Gerald Gastony, Director, Herbarium, IU
Dr. Loren Rieseberg, Genetics, IU

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Research Interests

The primary focus of my study is to investigate the genomic structure of a model fern species Ceratopteris richardii, and to understand the evolution of the homosporous fern genome.  Homosporous ferns, which produce only one kind of spores, are known to have high chromosome numbers compared to heterosporous plant species (e.g. flowering plants), which produce two kinds of spores.  For this reason, homosporous ferns are believed to be polyploids.  Their enzyme expressions, however, are generally of typical diploid organisms.  To resolve this discrepancy, Haufler (American Journal of Botany 74: 953-966) hypothesized that homosporous ferns are ancient polyploids (paleopolyploids) that have gone through gene silencing.  In my study, I intend to test the hypothesis of paleopolyploidy in the homosporous fern species, represented by Ceratopteris richardii, using genetic linkage mapping.  If it is a paleopolyploid, multiple copies of the majority of genes (paralogues) should exist in their genome with similar gene order.  The final linkage maps will reveal the number of paralogues, which suggest the minimum number of past polyploidization events, and the distribution of paralogues in the genome, which indicates the degree of genome rearrangements.

In addition, if Ceratopteris richardii is shown to be a paleopolyploid, then it should be feasible to test the duplicate gene silencing model of hybrid breakdown (Werth and Windham, American Naturalist 137: 515-526).  In this model, allopatric populations of a polyploid species “may experience silencing of the same gene but in different genomes” (Werth and Windham, American Naturalist 137: 515-526).  If many duplicated genes undergo reciprocal silencing, hybrids between these populations are likely to possess only silenced copies of some genes, causing inviability or sterility.  Lynch and Force (American Naturalist 156:590-605) have extended this model to allopatric diploid populations that contain duplicated genes.  They also note that subfunctionalization of genes rather than complete silencing should be sufficient for the development of hybrid breakdown.  One way to test the duplicate gene silencing model is to determine whether hybrid sterility is associated with paralogous loci.  If hybrid breakdown results from nonfunctional paralogous loci, then “complementary” genetic factors associated with breakdown should map to homoeologous chromosomes or chromosomal segments.

The resulting linkage maps of Ceratopteris richardii should improve our understanding of the genomic structure of an underrepresented plant group, homosporous species, and may reveal the mechanism of reproductive isolation.

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Publications

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