Chromosome-Level Genome Assembly and Annotation of a Periodical Cicada Species: Magicicada septendecula

We present a high-quality assembly and annotation of the periodical cicada species, Magicicada septendecula (Hemiptera: Auchenorrhyncha: Cicadidae). Periodical cicadas have a significant ecological impact, serving as a food source for many mammals, reptiles, and birds. Magicicada are well known for their massive emergences of 1 to 3 species that appear in different locations in the eastern United States nearly every year. These year classes ("broods") emerge dependably every 13 or 17 yr in a given location. Recently, it has become clear that 4-yr early or late emergences of a sizeable portion of a population are an important part of the history of brood formation; however, the biological mechanisms by which they track the passage of time remain a mystery. Using PacBio HiFi reads in conjunction with Hi-C proximity ligation data, we have assembled and annotated the first whole genome for a periodical cicada, an important resource for future phylogenetic and comparative genomic analysis. This also represents the first quality genome assembly and annotation for the Hemipteran superfamily Cicadoidea. With a scaffold N50 of 518.9 Mb and a complete BUSCO score of 96.7%, we are confident that this assembly will serve as a vital resource toward uncovering the genomic basis of periodical cicadas' long, synchronized life cycles and will provide a robust framework for further investigations into these insects.

Advances in the Evolution and Ecology of 13- and 17-Year Periodical Cicadas

Apart from model organisms, 13- and 17-year periodical cicadas (Hemiptera: Cicadidae: Magicicada) are among the most studied insects in evolution and ecology. They are attractive subjects because they predictably emerge in large numbers; have a complex biogeography shaped by both spatial and temporal isolation; and include three largely sympatric, parallel species groups that are, in a sense, evolutionary replicates. Magicicada are also relatively easy to capture and manipulate, and their spectacular, synchronized mass emergences facilitate outreach and citizen science opportunities. Since the last major review, studies of Magicicada have revealed insights into reproductive character displacement and the nature of species boundaries, provided additional examples of allochronic speciation, found evidence for repeated and parallel (but noncontemporaneous) evolution of 13- and 17-year life cycles, quantified the amount and direction of gene flow through time, revealed phylogeographic patterning resulting from paleoclimate change, examined the timing of juvenile development, and created hypotheses for the evolution of life-cycle control and the future effects of climate changeon Magicicada life cycles. New ecological studies have supported and questioned the role of prime numbers in Magicicada ecology and evolution, found bidirectional shifts in population size over generations, quantified the contribution of Magicicada to nutrient flow in forest ecosystems, and examined behavioral and biochemical interactions between Magicicada and their fungal parasites and bacterial endosymbionts.

Hybridization selects for prime-numbered life cycles in Magicicada: An individual-based simulation model of a structured periodical cicada population

We investigate competition between separate periodical cicada populations each possessing different life-cycle lengths. We build an individual-based model to simulate the cicada life cycle and allow random migrations to occur between patches inhabited by the different populations. We show that if hybridization between different cycle lengths produces offspring that have an intermediate life-cycle length, then predation acts disproportionately to select against the hybrid offspring. This happens because they emerge in low densities without the safety-in-numbers provided by either parent population. Thus, prime-numbered life cycles that can better avoid hybridization are favored. However, we find that this advantage of prime-numbered cycles occurs only if there is some mechanism that can occasionally synchronize emergence between local populations in sufficiently many patches.

Mitochondrial Genomics Reveals Shared Phylogeographic Patterns and Demographic History among Three Periodical Cicada Species Groups

The mass application of whole mitogenome (MG) sequencing has great potential for resolving complex phylogeographic patterns that cannot be resolved by partial mitogenomic sequences or nuclear markers. North American periodical cicadas (Magicicada) are well known for their periodical mass emergence at 17- and 13-year intervals in the north and south, respectively. Magicicada comprises three species groups, each containing one 17-year species and one or two 13-year species. Within each life cycle, single-aged cohorts, called broods, of periodical cicadas emerge in different years, and most broods contain members of all three species groups. There are 12 and three extant broods of 17- and 13-year cicadas, respectively. The phylogeographic relationships among the populations and broods within the species groups have not been clearly resolved. We analyzed 125 whole MG sequences from all broods and seven species within three species groups to ascertain the divergence history of the geographic and allochronic populations and their life cycles. Our mitogenomic phylogeny analysis clearly revealed that each of the three species groups had largely similar phylogeographic subdivisions (east, middle, and west) and demographic histories (rapid population expansion after the last glacial period). The mitogenomic phylogeny also partly resolved the brood diversification process, which could be explained by hypothetical temporary life cycle shifts, and showed that none of the 13- and 17-year species within the species groups was monophyletic, possibly due to gene flow between them. Our findings clearly reveal phylogeographic structures in the three Magicicada species groups, demonstrating the advantage of whole MG sequence data in phylogeographic studies.

Evolutionary Hysteresis and Ratchets in the Evolution of Periodical Cicadas

It has been previously hypothesized that the perfectly synchronized mass emergence of periodical cicadas (Magicicada spp.) evolved as a result of a switch from size-based to age-based emergence. In the former case, cicada nymphs emerge immediately (at the first opportunity) on reaching maturity, whereas in the latter case, nymphs wait in order to emerge at a specific age. Here we use an individual-based model to simulate the cicada life cycle and to study the evolution of periodicity. We find that if age-based emergence evolves in a constant abiotic environment, it typically results in a population that is protoperiodic, and synchronous emergence of the whole population is not achieved. However, perfect periodicity and synchronous emergence can be attained, if the abiotic environment changes back and forth between favorable and unfavorable conditions (hysteresis). Furthermore, once age-based emergence evolves, generally it can only be invaded by other age-based emergence strategies with longer cycle lengths (evolutionary ratchet). Together, these mechanisms promote the evolution of long periodic life cycles and synchronous emergence in the Magicicada. We discuss how our results connect to previous theories and recent phylogenetic studies on Magicicada evolution.

Triplicate parallel life cycle divergence despite gene flow in periodical cicadas

Periodical cicadas comprise three species groups containing three pairs of 13- and 17-year life cycle species showing parallel divergence, along with a more anciently diverged 13-year species (Magicicda tredecim). The mechanism and genetic basis of this parallel divergence is unknown. Here we use orthologous transcriptome sequences to explore the demographic processes and genomic evolution associated with parallel life cycle divergence. The three 13- and 17-year species pairs have similar demographic histories, and the two life cycles diverged 200,000-100,000 years ago. Interestingly, these life cycle differences have been maintained despite substantial gene flow between 13- and 17-year species within species groups, which is possible during co-emergences. Sequence divergence between 13- and 17-year species in each species group (excluding M. tredecim) is minimal, and we find no shared divergent single-nucleotide polymorphisms (SNPs) or loci associated with all instances of life cycle divergence. The two life cycles may be controlled by highly limited genomic differences.

Genomic divergence and lack of introgressive hybridization between two 13-year periodical cicadas support life cycle switching in the face of climate change

Life history evolution spurred by post-Pleistocene climatic change is hypothesized to be responsible for the present diversity in periodical cicadas (Magicicada), but the mechanism of life cycle change has been controversial. To understand the divergence process of 13-year and 17-year cicada life cycles, we studied genetic relationships between two synchronously emerging, parapatric 13-year periodical cicada species in the Decim group, Magicicada tredecim and M. neotredecim. The latter was hypothesized to be of hybrid origin or to have switched from a 17-year cycle via developmental plasticity. Phylogenetic analysis using restriction-site-associated DNA sequences for all Decim species and broods revealed that the 13-year M. tredecim lineage is genomically distinct from 17-year Magicicada septendecim but that 13-year M. neotredecim is not. We detected no significant introgression between M. tredecim and M. neotredecim/M. septendecim thus refuting the hypothesis that M. neotredecim are products of hybridization between M. tredecim and M. septendecim. Further, we found that introgressive hybridization is very rare or absent in the contact zone between the two 13-year species evidenced by segregation patterns in single nucleotide polymorphisms, mitochondrial lineage identity and head width and abdominal sternite colour phenotypes. Our study demonstrates that the two 13-year Decim species are of independent origin and nearly completely reproductively isolated. Combining our data with increasing observations of occasional life cycle change in part of a cohort (e.g. 4-year acceleration of emergence in 17-year species), we suggest a pivotal role for developmental plasticity in Magicicada life cycle evolution.

Evolution of periodicity in periodical cicadas

Periodical cicadas (Magicicada spp.) in the USA are famous for their unique prime-numbered life cycles of 13 and 17 years and their nearly perfectly synchronized mass emergences. Because almost all known species of cicada are non-periodical, periodicity is assumed to be a derived state. A leading hypothesis for the evolution of periodicity in Magicicada implicates the decline in average temperature during glacial periods. During the evolution of periodicity, the determinant of maturation in ancestral cicadas is hypothesized to have switched from size dependence to time (period) dependence. The selection for the prime-numbered cycles should have taken place only after the fixation of periodicity. Here, we build an individual-based model of cicadas under conditions of climatic cooling to explore the fixation of periodicity. In our model, under cold environments, extremely long juvenile stages lead to extremely low adult densities, limiting mating opportunities and favouring the evolution of synchronized emergence. Our results indicate that these changes, which were triggered by glacial cooling, could have led to the fixation of periodicity in the non-periodical ancestors.

Independent divergence of 13- and 17-y life cycles among three periodical cicada lineages

The evolution of 13- and 17-y periodical cicadas (Magicicada) is enigmatic because at any given location, up to three distinct species groups (Decim, Cassini, Decula) with synchronized life cycles are involved. Each species group is divided into one 13- and one 17-y species with the exception of the Decim group, which contains two 13-y species-13-y species are Magicicada tredecim, Magicicada neotredecim, Magicicada tredecassini, and Magicicada tredecula; and 17-y species are Magicicada septendecim, Magicicada cassini, and Magicicada septendecula. Here we show that the divergence leading to the present 13- and 17-y populations differs considerably among the species groups despite the fact that each group exhibits strikingly similar phylogeographic patterning. The earliest divergence of extant lineages occurred ∼4 Mya with one branch forming the Decim species group and the other subsequently splitting 2.5 Mya to form the Cassini and Decula species groups. The earliest split of extant lineages into 13- and 17-y life cycles occurred in the Decim lineage 0.5 Mya. All three species groups experienced at least one episode of life cycle divergence since the last glacial maximum. We hypothesize that despite independent origins, the three species groups achieved their current overlapping distributions because life-cycle synchronization of invading congeners to a dominant resident population enabled escape from predation and population persistence. The repeated life-cycle divergences supported by our data suggest the presence of a common genetic basis for the two life cycles in the three species groups.

Evolutionary ecology of periodical insects

To be periodical, a species must have a fixed life cycle length and adults must appear synchronously, reproduce only once, and die. The consequence of this life history is that, at a given location, adults of a periodical species will be absent or rare in some years and abundant in others. The relative scarcity of periodical Insect species suggests that periodicity does not evolve easily. The major obstacle to its evolution is selection favoring life cycles In which the offspring of any given female appear over a two- or three-year period. Chance events which disrupt this 'bet-hedging' strategy set the stage for periodicity. Mathematical models predict that, given certain initial conditions, intraspecific competition and predation favor its development. Recent studies suggest that periodicity is rarely perfect but that it can persist in the face of limited gene flow through time.

Permanence induced by life-cycle resonances: the periodical cicada problem

Periodical cicadas are known for their unusually long life cycle for insects and their prime periodicity of either 13 or 17 years. One of the explanations for the prime periodicity is that the prime periods are selected to prevent cicadas from resonating with predators with submultiple periods. This paper considers this hypothesis by investigating a population model for periodical predator and prey. The study shows that if the periods of the two periodical species are not coprime, then the predator cannot resist the invasion of the prey. On the other hand, if the periods are coprime, then the predator can resist the invasion of the prey. It is also shown that if the periods are not coprime, then the life-cycle resonance can induce a permanent system, in which no cohorts are missing in both populations. On the other hand, if the periods are coprime, then the system cannot be permanent.

Allee effect in the selection for prime-numbered cycles in periodical cicadas

Periodical cicadas are well known for their prime-numbered life cycles (17 and 13 years) and their mass periodical emergences. The origination and persistence of prime-numbered cycles are explained by the hybridization hypothesis on the basis of their lower likelihood of hybridization with other cycles. Recently, we showed by using an integer-based numerical model that prime-numbered cycles are indeed selected for among 10- to 20-year cycles. Here, we develop a real-number-based model to investigate the factors affecting the selection of prime-numbered cycles. We include an Allee effect in our model, such that a critical population size is set as an extinction threshold. We compare the real-number models with and without the Allee effect. The results show that in the presence of an Allee effect, prime-numbered life cycles are most likely to persist and to be selected under a wide range of extinction thresholds.

Selection for prime-number intervals in a numerical model of periodical cicada evolution

Periodical cicadas are known for unusually long and prime-numbered life cycles (13 and 17 years) for insects. To explain the evolution of prime-numbered reproductive intervals (life cycles), the hybridization hypothesis claims that prime numbers greatly reduce the chance of hybridization with other life cycles. We investigate the hybridization hypothesis using a simulation model. This model is a deterministic, discrete population model with three parameters: larval survival per year, clutch size, and emergence success. Reproductive intervals from 10 years to 20 years compete for survival in the simulations. The model makes three key assumptions: a Mendelian genetic system, random mating among broods of different life-cycle lengths, and integer population sizes. Longer life cycles have larger clutch sizes but suffer higher total mortality than shorter life cycles. Our results show that (1) nonprime-numbered reproductive intervals disappear rapidly in comparison to the selection among the various prime-numbered life cycles, (2) the selection of prime-numbered intervals happens only when populations are at the verge of extinction, and (3) the 13- and 17-year prime phenotypes evolve under certain conditions of the model and may coexist. The hybridization hypothesis is discussed in light of other hypotheses for the evolution of periodical cicada life cycles.

Evolutionary origin of whiteflies (Hemiptera: Sternorrhyncha: Aleyrodidae) inferred from 18S rDNA sequences

Phylogenetic analysis of 18S rDNA nucleotide sequences of hemipteran exemplars shows Sternorrhyncha (psyllids, whiteflies, aphids and scales) is monophyletic and forms a sister group to all other hemipterans (Euhemiptera). Whiteflies form a sister group to all other Sternorrhyncha. Primary structures of 18S rDNAs of all sternorrhynchans are exceptionally long (approximately 2200 to approximately 2500 bp) due to internal expansions. These expansions are a synapomorphy of Sternorrhyncha; other hemipterans possess shorter 18S rDNAs (approximately 1900 to approximately 1925 bp). The 18S rDNA of whiteflies is the longest recorded to date and has a base substitution rate of approximately 3 times greater than Euhemiptera taxa examined. The relevance of these findings to the fossil record, feeding strategies, reproductive biologies, and geoclimatic distribution is discussed.