Phenotypic variability and offspring fertility in Chelymorpha alternans
Chelymorpha alternans is a tortoise beetle distributed widely in Central and northern South America. The species on the Isthmus of Panama has five different morphotypes, differing strongly in the coloration of their elytra and pronotum. Differences in color pattern often serve as biological mating signals to conspecifics, or as warning signs of distastefulness or toxicity to predators in other insects. Preference for certain color patterns can drive divergent selection and lead to the speciation of sympatric populations. To begin to understand the role color variation plays in this species, we collected and mapped the relative distribution of each morph across the Isthmus. We performed crosses between five phenotypic variants and then backcrossed those progeny to most parental phenotypes. We then recorded the number and morphotype of offspring produced from each cross. Next, in order to generate an estimate of the geographic distribution and to gain an understanding of the degree of overlap between morph populations, 30 different sites around Panama were sampled, totaling 3,819 individuals. Individuals were assigned to one of 4 phenotypes based on elytra and pronotum coloration: “rufipennis”, metallic-striped “alternans”, “veraguensis”, and “darian”. The different morphotypes collected from each site where then plotted onto a map of Panama.
A distribution map created for C. alternans morphs in Panama shows that the “metallic” morph is widely distributed; occurring in nearly all collection sites. The “veraguensis” morph is found west of the canal on the Pacific side of Cordillera, and occurred at highest frequency (46%) in Guarumal. The “darian” morph is restricted to sites east of the canal and reaches greatest prevalence near the Colombian border. This morph contains two phenotypic variants, one with a striped pronotum and one with a black pronotum. The “rufipennis” morph is nearly as widespread as the “metallic”, but largely disappears near the Colombian border. Further studies are needed to determine how ecological factors may affect morphotype distribution. To determine how inheritance affects phenotypic expression, 19 crosses were performed in the lab between morphs, revealing that color pattern for this species follows basic Mendelian genetics. We find that the “metallic” is the only homozygotic morph; containing an “r” allele, recessive to the other color-determining alleles (R, T, and V) at the same locus. We also find that one of the “darian” morphs, is the result of a co-dominate state between the R and T alleles. The order of allelic dominance for this species is R>T>V>r.