Magicicada cassini

Agricultural Importance: Periodical cicadas have the longest life cycle from egg to adult of any insect and appear synchronously and periodically. They are probably the most unusual agricultural pest in that in any given orchard, adults appear only once every 13- or 17-years; however nearly every year periodical cicadas appear somewhere in the eastern US. Although infrequent in any one location, the effects are devastating. Female oviposition scars cause fruit laden branches to break. Millions of periodical cicada nymphs underground cause orchard decline. In forest ecosystems, periodical cicadas are major players in nutrient cycling and their periodic emergences provide a rich food source for above-ground predators.

Comparative Genomics: The infraordinal classification of Hemiptera (plant sucking bugs plus true bugs) remains in dispute. Several labs are collecting transcriptomic data for these insects and relatives (lice, plant lice, thrips, etc.) The majority of data comes from sternorrhynchan Hemiptera (aphids, scale insects, and relatives) and from Heteroptera (true bugs). Many fewer members of the clade Auchenorrhyncha have been targeted and all of these are leafhoppers (Membracoidea). The groups Cicadoidea (cicadas), Cercopoidea (spittlebugs), and Fulgoroidea (plant hoppers) have not been selected. Sequencing at least one member of each of these key hemipteran groups would be important for comparative purposes. The genomes of the two endosymbiont species obligate to Magicicada are being sequenced by John McCutcheon and he has volunteered to annotate the Magicicada genes involved in endosymbiont interactions.

Genome Size: From the little information we have to date to date, hemipteran genome sizes appear typical of hemimetabolous insects (> 2000 Mb). The exceptions are aphids (400-800 Mb) and the body louse (105 Mb). Periodical cicadas have quite large genomes 6400-6600 Mb (Johnston & Hanrahan, unpubl.). Ironically, cicada obligate endosymbionts have the smallest genomes known (144kb)! Spittlebugs are the sister group of cicadas and also have exceptionally large genomes. Could this have something to do with the fact that both groups feed on xylem fluid and are heavily dependent on endosymbionts for essential amino acids? It would be interesting to compare the genome size increase in these hemiptera to the similar phenomenon in Orthopteran insect orders.

Proposal: We suggest sequencing a trio of periodical cicada species for comparative purposes. The first two have already been suggested: Magicicada septendecim (Linnaeus’ 17-year cicada) and Magicicada tredecim (Riley’s 13-year cicada). We now suggest the third member of the trio, Fisher’s 17-year cicada (Magicicada cassini). Judging from mtDNA, this species is approximately 7-8% (uncorrected p-distance) from the –decim species (diverged ca. 3-4 Mya). These could be compared to a trio of Drosophila species with similar degrees of divergence.

Magicicada cassini (like its sibling species M. tredecassini) is slightly smaller than its congeners M. septendecim and M. tredecim. M. cassini is often found sympatrically with other species that share its lifecycle and year of emergence. In the Midwestern US, it is the most common periodical cicada and in Oklahoma and Texas, it is the only species of periodical cicada. Ecologically, it is most abundant in river valleys and other wet lowland forests. A comparison of the three species could reveal interesting aspects of the evolution of periodicity. For example, we hypothesize that the two M. -cassini species originated allopatrically from the M. -decim species and later joined pre-existing year-classes of the M. –decim species.