Hex Hox: De Novo Transcriptome Assembly and Embryonic Hox Gene Expression in the Burrowing Mayfly Hexagenia Limbata (Ephemeridae)

Background: Hox genes are key regulators of appendage development in the insect body plan. The body plan of Mayfly (Ephemeroptera) nymphs differs due to the presence of evolutionarily significant abdominal appendages called gills. Despite mayflies’ basal phylogenetic position and novel morphology amongst insects, little is known of their developmental genetics. Here we present an annotated transcriptome for the mayfly Hexagenia limbata, with annotated sequences for putative Hox peptides and embryonic expression profiles for the Hox genes Antp and Ubx/abd-A. Results: Transcriptomic sequencing of early instar H. limbata nymphs yielded a high-quality assembly of 83,795 contigs, of which 22,975 were annotated against Folsomia candida, Nilaparvata lugens, Zootermopsis nevadensis and UniRef90 protein databases. Peptide annotations included eight of the ten canonical Hox genes (lab, pb, Dfd, Scr, Antp, Ubx, abd-A and Abd-B), most of which contained all functional domains and motifs conserved in insects. Expression patterns of Antp and Ubx/abd-A in H. limbata were visualized from early to late embryogenesis, and are also highly conserved with patterns reported for other non-holometabolous insects.Conclusions: We present evidence that both H. limbata Hox peptide sequences and embryonic expression patterns for Antp and Ubx/abd-A are extensively conserved with other insects. These findings suggest mayfly Antp and Ubx/abd-A play similar appendage promoting and repressing roles in the thorax and abdomen, respectively. The identified expression of Ubx and abd-A in early instar nymphs further suggests that mayfly gill development is not subject to Ubx or abd-A repression. Previous studies have shown that insect Ubx and abd-A repress appendages by inhibiting their distal structures, which can permit the development of proximal appendage types. In line with past morphology-based work, we propose that mayfly gills are proximal appendage structures, possibly homologous to the proximal appendage structures of crustaceans.

A preliminary molecular assessment of the taxonomic validity of Hexagenia orlando Traver, 1931 (Ephemeroptera: Ephemeridae)

In an effort to evaluate the taxonomic uncertainty of the species status of Hexagenia orlando Traver, 1931 (Ephemeroptera: Ephemeridae), molecular data were used for the first time to test its current classification. Mitochondrial cytochrome c oxidase subunit I (COI) haplotypes were evaluated using three types of analyses in the form of distance-based, tree-based, and model-based delimitation methods. All analyses consistently recovered H. orlando as a valid species. These preliminary results lend evidence reinforcing the current classification scheme and encourage renewed scrutiny of adults and nymphs to hopefully identify additional morphological characters that may serve to separate H. orlando and its congener Hexagenia limbata (Serville, 1829).

Giant Burrowing Mayfly (suggested common name) Hexagenia limbata (Serville 1829) (Insecta: Ephemeroptera: Ephemeroidea: Ephemeridae)

The giant burrowing mayfly, Hexagenia limbata (Serville, 1829) (Figure 1) is one of the most widespread mayflies in North America and is well known for its importance in ecosystem health and water quality monitoring. The mayflies (order Ephemeroptera) are an ancient lineage of aquatic insects originating more than 300 million years ago (Merritt and Cummins 2008). Within this order, the burrowing mayflies of the family Ephemeridae are well-known for their importance in fly fishing and their massive synchronized mating flights.https://edis.ifas.ufl.edu/in1244 Also published on the Featured Creatures website: http://entnemdept.ufl.edu/creatures/AQUATIC/mayfly.html

A Tribute to Richard A. Vollenweider (1922-2007)

This study evaluates the suitability of using Hyalella azteca as a predictor of the risk that tributyltin (TBT) poses to freshwater invertebrates by comparing the toxicity and bioaccumulation of TBT in H. azteca to five species: Hexagenia limbata, Physella gyrina, Tubifex tubifex, Chironomus riparius and Daphnia magna. Young from each species were added to aquaria containing sediment spiked with TBT concentrations of 0, 28.6, 258 and 1900 ng Sn/g dry weight. Chironomus riparius data could not be quantified reliably due to poor control survival. Bioaccumulation of TBT in the five remaining species was similar at sediment concentrations of 258 ng Sn/g, and the bioaccumulation relationships of P. gyrina and H. azteca were virtually identical. No toxicity was evident in any test species at body concentrations of 1100 to 1800 ng Sn/g. Body concentrations of 4000 to 6000 ng Sn/g were associated with significant (p < 0.05) mortality in H. limbata, P. gyrina, T. tubifex and D. magna, but not in H. azteca. However, bioaccumulation of TBT in H. azteca was comparable and can be used in predicting the effects of environmental TBT concentrations on these four species of freshwater invertebrates.