The Effect of Chromosomes on Courtship Behavior in Sibling Species of the Drosophila virilis Group

Simple Summary Courtship behavior has been the focus of research for a long time due to its association with one of the earliest and most effective mechanisms of interspecies isolation. The Drosophila virilis species group, which is a model for studying the mechanisms of evolution, has been studied extensively for various aspects of the courtship ritual, including the male courtship song. Despite genetic fine-mapping of this courtship behavior element, no genetic analysis of the ritual as a whole has been conducted. This study aimed to determine the effect of chromosomes on the latency and duration of courtship-ritual elements in the D. americana and D. virilis species system. Since the expression of the courtship-ritual structure depends on the behavior of both the female and the male, to study the inheritance of the courtship elements’ parameters, tests were carried out with reference partners (females and males of the original species) of representatives of different generations: parental, F1, F2, and backcrosses (FB). Structural equation modeling was used to process the parameter data for a series of courtship-ritual elements. The obtained results indicated that species-specific variability in courtship-element parameters in females is concentrated on the autosomes, while in males, it is concentrated on the X chromosome. This influence of the X chromosome in successful male courtship behavior may be a contributing factor in the lack of complete X chromosome demasculinization observed in Drosophila. Abstract Prezygotic isolation mechanisms, particularly courtship behavior, play a significant role in the formation of reproductive barriers. The action of these mechanisms leads to the coexistence of numerous closely related insect species with specific adaptations in a shared or adjacent territory. The genetic basis of these mechanisms has been studied using closely related Drosophila species, such as the D. virilis group. However, the investigation of individual courtship behavior elements has been limited until recently, and the effect of genotype on the species-specific features of courtship as a whole has not been thoroughly examined. It should be noted that courtship behavior is not a typical quantitative trait that can be easily measured or quantified in both females and males, similar to traits like wing length or bristle number. Each courtship element involves the participation of both female and male partners, making the genetic analysis of this behavior complex. As a result, the traditional approach of genetic analysis for quantitative traits, which involves variance decomposition in a set of crosses, including parental species, F1 and F2 hybrids, and backcrosses of F1 to parental species, is not suitable for analyzing courtship behavior. To address this, we employed a modified design by introducing what we refer to as ‘reference partners’ during the testing of hybrid individuals from F1, F2, and backcrosses. These reference partners represented one of the parental species. This approach allowed us to categorize all possible test combinations into four groups based on the reference partner’s sex (female or male) and their constant genotype towards one of the parental species (D. virilis or D. americana). The genotype of the second partner in the within-group test combinations varied from completely conspecific to completely heterospecific, based on the parental chromosomal sets. To assess the contribution of partner genotypes to the variability of courtship-element parameters, we employed structural equation modeling (SEM) instead of the traditional analysis of variance (ANOVA). SEM enabled us to estimate the regression of the proportion of chromosomes of a specific species type on the value of each courtship-element parameter in partners with varying genotypes across different test combinations. The aim of the current study was to analyze the involvement of sex chromosomes and autosomes in the formation of courtship structure in D. virilis and D. americana. The genetic analysis was complemented by video recording and formalization of courtship-ritual elements. D. virilis was found to be more sensitive to mate stimuli compared to D. americana. The majority of species-specific parameters, such as latency and duration of courtship elements (e.g., male and female song, following, licking, and circling), were shown to be influenced by the D. virilis genotype. However, not all of these parameters significantly impact copulation success, with the male song, licking, and following being the most significant. In females, the female song was found to have a significant relationship only with copulation duration. The influence of the female genotype on the species-specific parameters of courtship elements is primarily related to autosomes, while the male genotype is associated with the X chromosomes. The study suggests that sexual selection primarily occurs through acoustic and chemoreceptor channels.