Physiological and life history changes associated with seasonal adaptation in the cactophilic Drosophila mojavensis

Many insects inhabiting temperate climates are faced with changing environmental conditions throughout the year. Depending on the species, these environmental fluctuations can be experienced within a single generation or across multiple generations. Strategies for dealing with these seasonal changes vary across populations. Drosophila mojavensis is a cactophilic Drosophila species endemic to the Sonoran Desert. The Sonoran Desert regularly reaches temperatures of 50°C in the summer months. As individuals of this population are rare to collect in the summer months, we therefore simulated in a temperature and light controlled chamber the cycling temperatures experienced by D. mojavensis in the Sonoran Desert from April to July (four generations) to understand the physiological and life history changes that allow this population to withstand these conditions. In contrast to our hypothesis of a summer aestivation, we found that D. mojavensis continue to reproduce during the summer months, albeit with lower viability, but the longevity of the population is highly reduced during this period. As expected, stress resistance increased during the summer months in both the adult and the larval stages. This study examines several strategies for withstanding the Sonoran Desert summer conditions which may be informative in the study of other desert endemic species.

An Improved Genome Assembly for Drosophila navojoa, the Basal Species in the mojavensis Cluster

Three North American cactophilic Drosophila species, D. mojavensis, D. arizonae, and D. navojoa, are of considerable evolutionary interest owing to the shift from breeding in Opuntia cacti to columnar species. The 3 species form the “mojavensis cluster” of Drosophila. The genome of D. mojavensis was sequenced in 2007 and the genomes of D. navojoa and D. arizonae were sequenced together in 2016 using the same technology (Illumina) and assembly software (AllPaths-LG). Yet, unfortunately, the D. navojoa genome was considerably more fragmented and incomplete than its sister species, rendering it less useful for evolutionary genetic studies. The D. navojoa read dataset does not fully meet the strict insert size required by the assembler used (AllPaths-LG) and this incompatibility might explain its assembly problems. Accordingly, when we re-assembled the genome of D. navojoa with the SPAdes assembler, which does not have the strict AllPaths-LG requirements, we obtained a substantial improvement in all quality indicators such as N50 (from 84 kb to 389 kb) and BUSCO coverage (from 77% to 97%). Here we share a new, improved reference assembly for D. navojoa genome, along with a RNAseq transcriptome. Given the basal relationship of the Opuntia breeding D. navojoa to the columnar breeding D. arizonae and D. mojavensis, the improved assembly and annotation will allow researchers to address a range of questions associated with the genomics of host shifts, chromosomal rearrangements and speciation in this group.