Hybrid Classification: Insights From Genetic Map-Based Studies of Experimental Hybrids

Correct interpretations of hybridization phenomena rely on accurate methods of detecting hybrids and estimating genotypic classes. Multilocus molecular data sets have proved to be particularly useful for hybrid identification and genealogical reconstruction. One widely recognized limitation to this approach is that hybrid and parental genotypic classes often differ minimally in terms of expected marker numbers, and an extremely large number of molecular markers are required to distinguish between them. A more fundamental difficulty, and the focus of this study, concerns the potential for selection to bias marker numbers in hybrids, thus leading to faulty genealogical assignments. In this paper, we compare observed and expected marker numbers in 170 hybrids of known pedigree between two wild sunflower species, Helianthus annuus and H. petiolaris. All 170 hybrid plants had fewer H. petiolaris markers than the expected means. In fact, with the exception of two individuals, marker numbers were more consistent with pedigrees involving three to seven backcrossed generations, rather than the two generations of backcrossing actually employed. Although the implications of these data for genealogical reconstruction are discouraging, it is the actual genetic constitution of hybrids, not the pedigree, that is most predictive of their characteristics or behavior. Thus, we suggest that hybrids be classified either in terms of genetic relatedness or in terms of the admixture of parental-species genes they carry. Both represent simple approaches for generating biologically cohesive categories for ecological or comparative studies.