GARDNER, KEITH A.1*, STUART J.E. BAIRD2, SHANNA E. CARNEY3, and LOREN H. RIESEBERG1. 1Department of Biology, Indiana University, Bloomington IN 47405; 2University of Queensland, Brisbane, Australia; 3Department of Biology, Colorado State University, Fort Collins, Colorado 80523. - A comparison of different molecular and morphological markers for hybrid index estimation in a natural hybrid zone.
Areas of hybridization between related species have become fertile
ground for studying the evolutionary process at the genetic level. To
fully exploit the opportunities offered by hybrid zones, we need a
reliable index of the hybridity of individual organisms. We have
developed maximum likelihood methods for the analysis of molecular and
morphological marker data, both for codominant and dominant markers
and for qualitative and quantitative morphological traits. In
simulation studies, we looked at the effects of marker number, marker
diagnosticity, linkage and allelic dominance relations on these
estimates and their support limits (confidence levels). In addition,
for dominant markers, we examined the degree to which violation of
Hardy-Weinberg equilibrium in the parental populations biases
estimates. We tested the predictions of the simulation study by
analysis of a large data set (304 hybrid individuals) from a natural
hybrid zone between Helianthus annuus and Helianthus
bolanderi , for which we screened microsatellite (codominant
molecular), AFLP (dominant molecular) and morphological marker traits.
Comparisons of hybrid indices based on these three kinds of markers
generally confirm our earlier simulation conclusions. Over most of the
range of possible hybrid index values,a given number of codominant
markers give significantly, but not dramatically, better confidence
intervals than dominant markers. For most hybrid zone studies, about
50 dominant or 30-40 codominant markers would produce acceptable
confidence intervals, assuming that they are non-diagnostic but do
show significant differences in frequency between the parental
populations. The hybrid index estimates are also very robust to
deviations of FIS from 0. The codominant
microsatellite-based hybrid index scores were significantly better
predictors of morphology-based hybrid index scores
(R2=0.65) than were the AFLP hybrid index scores
R2=0.34). Surprisingly, the codominant index was a poorer
predictor of the AFLP index than it was of the morphology based
index.We are currently investigating whether these discrepancies have
a methodological or biological explanation.
Key words: Helianthus annuus, Helianthus bolanderi, Hybrid index, Hybrid zone, microsatellite, Molecular markers