Additive Expression Suggests Mendelian Inheritance of Polymorphic Venom Phenotypes in Mojave Rattles
Department of Biological Sciences
Clemson University Clemson, South Carolina, USA
The combination of phenotypes inherited from parents partially determines fitness of an individual. Monomorphic phenotypes under strong selection tend to move to fitness optima but polymorphic phenotypes that can be inherited in different combinations will result in offspring with fitness that differs from one or both parents depending on the complexity of the genotype to phenotype pathway. Venom is a polymorphic phenotype that is under strong selection and highly tractable from the genotype to phenotype. In rattlesnakes, particularly Mojave Rattlesnakes (Crotalus scutulatus), a venom dichotomy exists. This dichotomy was initially defined based on proteomic characteristics but recently, a genomic definition has been proposed. Some C. scutulatus have neurotoxic venom that lacks hemorrhagic activity (Type A) and others have high hemorrhagic activity and no neurotoxic activity (Type B). Rarely, individuals have a third venom type, Type A+B, where they have neurotoxic and hemorrhagic venom. We used the proteomic definition to identify putative Type A+B C. scutulatus to test the applicability of the genomic definition of venom type. To do this, we used comparative transcriptomics of the venom-gland on 31 C. scutulatus and tested for differential expression between venom types. We then applied the genomic definition and were able to identify the genotype for the phospholipases and snake venom metalloproteinases responsible for the venom phenotypes. The heterozygous individuals expressed toxins at approximately half of the level found in homozygous individuals. Additive expression in the eight toxins tested suggests the phenotypes in C. scutulatus are inherited in a Mendelian fashion and cis-regulation is responsible for differences in expression. We identified six of the nine possible genotypes for a dihybrid cross including five from at least the F2 generation. We were able to determine that Type A+B venom is not unique and is generated through interbreeding between Type A and Type B individuals.