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Hybridization and the genetics of wing colour-pattern diversity in Heliconius butterflies

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Salazar Carrión, Patricio Alejandro 


Diversity is perhaps , the most outstanding feature of the living world. Traditionally, biological diversity was thought to arise mainly through de nova mutations that increase in frequency through natural selection or genetic drift. However, it is now recognized that hybridization between already divergent populations constitutes another important source of genetic and phenotypic variation, upon which natural and/or sexual selection can operate. Hence, the idea that hybridization can actually promote diversification is becoming increasingly supported. In this thesis work, I explore the role of hybridization in the origin of biological diversity. In particular, I have studied the structure of two overlapping hybrid zones between races of two mimetic species of Heliconius butterflies: Heliconius melpomene and H. erato, in Eastern Ecuador, South America. This thesis work is composed of four empirical studies: 1) a characterization of the phenotypic effects caused by the major wing colour-pattern loci that segregate in the studied hybrid zones, 2) a characterization of the hybrid zones in the field, 3) a field experiment to test whether frequency-dependent selection is maintaining the distinctiveness of the hybridizing races in spite of gene flow, and whether it explains the high abundance of one particular H. erato hybrid form, and 4) a quantification of continuous colour-pattern variation using a novel morphometric technique, which was then used for a preliminary QTL mapping analysis of colour-pattern at the chromosome level. The most outstanding finding of this thesis was the fact that the allele frequency dines for the two major colour-pattern loci segregating in the hybrid zone (D and Ac/ScfJ are significantly displaced from each other in both studied species. This dine displacement is associated with the high abundance of one particular hybrid form of H. erato. This hybrid phenotype is one of the possible homozygote hybrids (i.e. homozygote for the allele from one of the parental populations at one locus, but homozygote for the allele of the other parental population at the other locus). As a result, this hybrid breeds true when mated to its own kind, and could potentially form a self-sustaining population. It was hypothesised then that this particularly abundant hybrid phenotype might constitute a diverging hybrid form (i.e. a sort of incipient race). The implications of this hypothesis led to the field � � . experiment aimed at testing whether frequency-dependent selection was favouring the highly abundant hybri'd phenotype in the zone of cline mismatch. The results of this experiment were mixed but enlightening: even though I could not detect differences in attack rates associated with colour-pattern, I found that all tested colour morphs were significantly more attacked in the hybrid zone than in the parental .zones. This finding suggested that predation against all colour morphs is stronger in t~e hy�b�nd ~one. The implications of all these results for the role of hybridization in biological d1vers1f1cat10n are discussed extensively in this thesis. By studying the genetic bases of colour-pattern as well as the dynamics of hybridization in the wild, this thesis work is making important contributions to the understanding of the role of hybridization in biological diversification including thee reconstructin of a possible route through which novel hybrid phenotypes could be established in nature.


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Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge