1. ¹Department of Biology, Sonoma State University, Rohnert Park, CA 94928, USA
2. ²Department of Botany and Plant Sciences, University of California, Riverside, CA 92521, USA
3. ³Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA

Correspondence: Richard Whitkus, E-mail: whitkus@sonoma.edu

Received 3 September 1999; Accepted 31 January 2000.

Abstract

Despite numerous studies of speciation on oceanic islands, few insights exist on the genetic changes involved in the origin and diversification of island taxa. Here we report a genetic analysis of the evolutionary change in sex expression in Hawaiian Tetramolopium. The most diverse clade in the genus is characterized by a monoecious breeding system. The breeding system resulted from a change in sex expression in disc florets from the ancestral hermaphroditic condition to the derived male state. Analysis of an F₂ population from a cross between the two forms of sex expression indicates two regions of the Tetramolopium linkage map are associated with the loss of female function in disc florets. Quantitative trait locus mapping of the two linkage groups confirms that two loci control 56% of the phenotypic variation of the trait in the F₂ population. Additive and dominance effects are apparent but no statistical evidence of epistasis was found. Several related reproductive traits also have few genetic associations on the linkage map, but are generally distinct from the control of sex expression. Although modifier loci are likely to be involved, the apparent simple genetic change underlying sex expression parallels a major evolutionary diversification in Hawaiian Tetramolopium and may have initiated the divergence of this novel clade.