Joshua R. Kohn
 Professor of Biology, UCSD
Chair, Ecology, Behavior & Evolution Section

e-mail: jkohn@ucsd.edu
lab homepage: http://biology.ucsd.edu/labs/kohn

     We use both molecular and experimental tools to study plant mating systems and life-history evolution. Current work focuses on three issues: 1) The molecular evolution of the self-incompatibility locus, 2) The genetic basis of life-history variation between annual and perennial ecotypes of the same species, and 3) The evolution of mating system diversity in higher plants. 

SELF-INCOMPATIBILITY IN THE SOLANACEAE
      Many plants reject their own pollen to avoid the detrimental effects of inbreeding. Self pollen rejection is often controlled by a single locus (S). If the allele carried by the pollen matches either allele in the female parent, pollen tube growth is arrested. Rare alleles at this locus have a selective advantage, being compatible with more potential mates. This frequency-dependent selection leads to some of the highest levels of polymorphism known for any locus with 30-50 alleles often segregating within single populations. The number of alleles maintained in a population provides a genetic means of estimating effective population size.  At the molecular level allelic lineages are often tens of millions of years old, older than the species in which they currently reside.  This is reflected in the fact that an allele found in one species is often more closely related to an allele found in another species than it is to other alleles from the same species. Because of this property, the S-locus provides a tool for historical inference that extends much deeper in time than neutral variation. 

      Using RT-PCR to amplify S-alleles from stylar tissue, we can rapidly survey S-allele diversity within and between natural populations and simultaneously gain sequence information that can be used to study evolutionary processes above the species level. At the ecological level, we use this locus to study the relationship between the ecological characteristics of species and their effective population size, a parameter of fundamental importance to evolution and conservation. The locus is also useful for
detecting the frequency of population restrictions that occurred millions of years in the past. Such restrictions are required for founder event speciation models, so examination of the S-locus allows the frequency of this mode of speciation to be assessed.  Finally, we uncover closely related sequences which may lead to an understanding of the relationship between sequence differences and rejection specificity. 

THE GENETIC ARCHITECTURE OF ADAPTIVE QUANTITATIVE VARIATION
      We use molecular markers to map and characterize the phenotypic effects of loci controlling developmental differences between annual and perennial ecotypes of Oryza rufipogon, the wild progenitor of rice. Annual ecotypes have high yield, low vegetative survival and selfing flowers while perennial ecotypes have low fecundity, high vegetative persistence, and more outcrossing flowers. These trait syndromes are predicted by evolutionary life history theory, but little is known about the genetic basis of life-history differences. Genes that control the differences between annual and perennial growth patterns alter the way plants allocate resources between vegetative growth reproductive structures. Using quantitative trait locus analysis we ask  whether such differences are controlled by few or many genes and whether the same loci have caused ecotypic differentiation in different geographical regions. 

THE EVOLUTION OF PLANT MATING SYSTEMS
      Flowering plants exhibit far more breeding system diversity than do vertebrate animals. We use both experimental and phylogenetic approaches to test evolutionary hypotheses concerning mating system diversity. We are particularly interested in the evolution of separate sexes vs. hermaphroditism and self-fertilization vs. outcrossing. Recent work focuses on two unusual systems: gynodioecy (females and hermaphrodites) and tristyly (three distinct hermaphroditic morphs), each of which has
evolved independently many times in higher plants. Because mating traits have strong effects on fitness, polymorphic systems such as these provide unusually good opportunities to study the mechanics of natural selection. 

     Streisfeld, MA, and JR Kohn. (2006). Environment and pollinator-mediated selection on parapatric floral races of Mimulus aurantiacus. Journal of Evolutionary Biology (in press).

     Igic B, and JR Kohn. (2006). The distribution of plant mating systems: study bias against obligately outcrossing species. Evolution 60: 1098-1103.

     Igic, B, L Bohs, and JR Kohn. (2006). Molecular fossils reveal unidirectional breeding system shifts. Proceedings of the National Academy of Sciences, USA 103:1359-1363.

     Streisfeld, MA, and JR Kohn. (2005). Contrasting patterns of floral and molecular variation across a cline in Mimulus aurantiacus. Evolution 59:2548-2559.

     Yeung K., JS Miller, AE Savage, BC Husband, B Igic  and JR Kohn. (2005).  Association of ploidy and sexual system in Lycium californicum (Solanaceae). Evolution 59: 2048-55.

     Igic, B. and J. R. Kohn. (2001). Evolutionary relationships among self-incompatibility RNases. Proceedings of the National Academy of Sciences, USA 98:13167-13171. 
     Joshua R. Kohn received his Ph.D. from the University of Pennsylvania and was a postdoctoral fellow at both the University of Toronto and Cornell University.