Introgressing Pheromone QTL Between Species: Towards an Evolutionary Understanding of Differentiation in Sexual Communication

Abstract

Abstract-As a first step toward understanding how noctuid moths evolve species-specific pheromone communication systems, we hybridized and backcrossed two closely related moth species, Heliothis virescens (Hv) and H. subflexa (Hs), which differ qualitatively and quantitatively in their multicomponent sex pheromone blends. We used amplified fragment length polymorphism (AFLP) marker-based mapping of backcross families to determine which of the 30 autosomes in these moths contained quantitative trait loci (QTL) controlling the percentages of specific chemical components in the pheromone blends. In two previous backcrosses to Hs, we found a strong depressive effect of Hv-chromosome 22 on the percentage of three acetate components in the pheromone gland. These acetates are present in Hs and absent in Hv. Here, we describe how we introgressed Hv-chromosome 22 into the genomic background of Hs. Selection for Hv-chromosome 22 started from backcross 3 (BC 3 ) females. All females that had Hv-chromosome 22 and a low percentage of acetates (<3% of the total amount of pheromone components present) were backcrossed to Hs males. In BC 5 to BC 8 , we determined whether Hvchromosome 22 was present by a) running only the primer pairs that would yield the markers for that chromosome, and/or b) determining the relative percentages of acetates in the pheromone glands. Either or both genotype and phenotype were used as a criterion to continue to backcross these females to Hs males. In BC 9 , we confirmed the isolation of Hv-chromosome 22 in the Hs genomic background, and backcrossed the males to Hs females to eliminate the Hv-sex chromosome as well as mitochondrial DNA. The pheromone composition was determined in BC 3 , BC 5 , and BC 11 females with and without * To whom correspondence should be addressed. E-mail: astrid groot@ncsu.edu Hv-chromosome 22. All backcross females with Hv-chromosome 22 contained significantly less acetates than females without this chromosome. In addition, BC 3 females with Hv-chromosome 22 contained significantly more Z11-16:OH than BC 3 females without Hv-chromosome 22. However, in BC 5 and BC 11 females, the correlation between Z11-16:OH and Hv-chromosome 22 was lost, suggesting that there are separate QTL for the acetates and for Z11-16:OH, and that the relative amount of the alcohol component is only affected in epistasis with other (minor) QTL. Now that we have succeeded in isolating the chromosome that has a major effect on acetate production, we can test in behavioral experiments whether the presence of acetates may have been a driving force for a shift in pheromone composition. Such tests are necessary to move towards an evolutionary understanding of the differentiation in sexual communication in Heliothis spp. moths.

On the scent of speciation: the chemosensory system and its role in premating isolation

“…Finally, quantitative variation of the principal CHC, (Z)-11-pentacosene, between a genetically marked strain of D. virilis and D. novamexicana segregated with two of the six chromosomes and one major genetic factor was mapped (Doi et al, 1996; for a review of Drosophila work, see Ferveur, 2005;Gleason et al, 2005). This polygenic basis is also found for female differences in long-range pheromone compounds between H. virescens and H. subflexa (Groot et al, 2004;Sheck et al, 2006).…”

Experimental evidence for interspecific directional selection on moth pheromone communication

“…The relative amounts of the other five pheromone components that differ between the two species showed the Hs-like pattern and were not significantly different between the backcross females with the norm-OAc QTL and those with the low-OAc QTL (see also ref. 55). …”

“…As the relative amount of acetates declined in backcross Hs females with the low-OAc QTL, they surprisingly attracted more Hv males. Although the addition of Z11-16:OAc, the major acetate component in Hs females, to a synthetic Hv pheromone blend has been shown to suppress upwind flight of Hv males (54), we had expected species-specificity to remain encoded in the other five Hs pheromone gland components that were equally represented in norm-and low-OAc Hs females, because the relative amounts of these components differ substantially from the Hv pheromone blend (55,56). (Indeed, Hv females that do not produce any component that is behaviorally antagonistic to Hs males attracted 7.9 Hv males per female per night and only 0.08 Hs males per female per night in the field.)…”

“…We introgressed one quantitative trait locus (QTL) for low production of the acetate compounds (which we denote lowOAc QTL) into an Hs genomic background (55,56). We then conducted field and cage experiments with the backcross female offspring [with the introgressed low-OAc QTL and without (norm-OAc QTL)] to measure their relative capacity to attract and mate with Hs and Hv males.…”

Sexual isolation of male moths explained by a single pheromone response QTL containing four receptor genes

“…To further examine the role of genes within C27 in male response, Hs-C27 was introgressed into the Hv genomic background by repeatedly backcrossing females with Hs-C27 to Hv males (for general methods see refs. 28,48). This resulted in a BC line that was identical to Hv except that about 1/2 of the individuals inherited one copy of Hs-C27 (checked with AFLPs and HR14 sequences).…”