Physella acuta ecogenomics and snail population fitness

Coen M. Adema

Abstract:

The freshwater snail Physella acuta has achieved global distribution, extending far beyond the native range in North America. Phylogenetic studies indicated that invasive P. acuta represent a subpopulation within the species with a mitogenome haplotype A that displays ~10% sequence difference from mitohaplotype B, present in the native range. To study the underlying biology, field collected (NM) snails were genetically characterized to initiate uni-maternal lab-populations of P. acuta with haplotypes A and B. The fitness (growth rate, age to maturity, reproductive output, survival) of these populations was compared for snails maintained under constant (lab-)conditions. A rewilding approach was employed also, exposing lab-reared adult snails in cages to naturally variable conditions at field sites and comparing fitness with lab-maintained snails over 1- or 2-week intervals. The natural digenetic trematodes parasite fauna of P. acuta was surveyed to develop a PCR screening assay for field-exposed snails that incurred infections. The fitness of populations A and B did not differ for P. acuta held under lab conditions (7 experiments). In case of rewilding, population A had greater fitness (survival, individual egg mass production) than population B in 3 out of 7 experiments. For a 1-week experiment (fitness A > B), RNAseq (Illumina) was performed to compare gene expression profiles from individual snails of populations A and B, lab maintained and rewilded. After removing rDNA- and mitogenome-derived sequences, Trinity de novo assembly yielded a A+B reference transcriptome (377492 transcripts > 500nt, N50:1074, BUSCO >95%). Ongoing differential expression analyses showed modest changes in transcriptional profiles of population B snails between lab and field. By comparison, P. acuta of population A displayed a greater transcriptional diversity, involving several biological processes, including immunity. This ecogenomics approach may provide a fingerprint for increased fitness (invasive potential) of populations within the species P. acuta.