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Calycadenia : a model system for investigating chromosomal speciation Olson, Teika E.


The theory of chromosomal speciation proposes that cytological differences such as reciprocal translocations and pericentric inversions may present the primary barrier to gene flow among diverging groups. While theory supports the possbility of chromosomal speciation, conclusive evidence from nature is lacking and few natural systems have been developed for investigating the phenomenon. The California tarweed species Calycadenia fremontii A. Gray and C. paucijlora A. Gray provide an ideal system in which to examine some of the predictions of chromosomal speciation. These species comprise numerous cytological races distinguished primarily by reciprocal translocations. Cytological differences among the races correlate with reduced fertility among hybrid Fl progeny, suggesting a possible isolating role for chromosomal rearrangements in this group. To test whether cytological differences among populations have played a primary role in driving divergence, genetic distances among 460 individuals collected from 23 populations were calculated from allele frequencies based on 88 polymorphic RAPD markers shared among populations. Genetic distances were used to construct Neighbor-Joining and Fitch-Margoliash trees from which past patterns of gene flow among populations and cytological races could be inferred. As an alternative method for estimating genetic divergence among populations and cytotypes, allele frequencies estimated from RAPD markers were also used to measure centroid F[sub ST] for each population. Both the genetic distance trees and centroid F[sub ST] statistics indicate that populations are genetically differentiated. Futhermore, cytological races are not resolved into monophyletic clades in the genetic distance trees suggesting that cytological differences among populations do not account for the primary barrier to gene flow among them. Mantel tests confirm that cytological differences among populations do not explain the variation in genetic distances among populations, although geography does. I conclude that cytological differences among populations in Calycadenia fremontii and C. pauciflora have not presented the primary barrier to gene flow, and find that the theory of chromosomal speciation is not supported.

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