Multiple differences in calling songs and other traits between solitary and gregarious Mormon crickets from allopatric mtDNA clades

Nathan W Bailey,William V Bailey,Darryl T Gwynne,Michael G Ritchie

doi:10.1186/1471-2148-7-5

Nathan W Bailey, William V Bailey + Show 2 more

Open Access

https://doi.org/10.1186/1471-2148-7-5

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Abstract

BackgroundIn acoustic species, traits such as male calling song are likely to diverge quickly between allopatric populations due to sexual selection, and divergence in parameters such as carrier frequency, chirp structure, and other important song characters can influence sexual isolation. Here we make use of two forms of Mormon crickets to examine differences in a broad suite of traits that have the potential to influence speciation via sexual isolation. Mormon crickets in "gregarious" populations aggregate into dense migratory bands, and females are the sexually competitive sex (sex-role reversal). There is also a non-outbreak "solitary" form. These two forms are largely but not perfectly correlated with a significant mtDNA subdivision within the species that is thought to have arisen in allopatry. Combined information about multiple, independently evolving traits, such as morphology and structural and behavioural differences in calling song, provides greater resolution of the overall differences between these allopatric populations, and allows us to assess their stage of divergence. We test two predictions, first that the forms differ in song and second that gregarious males are more reluctant to sing than solitary males due to sex role reversal. We also tested for a difference in the relationship between the size of the forewing resonator, the mirror, and carrier frequency, as most models of sound production in crickets indicate that mirror size should predict carrier frequency.ResultsMultivariate analyses showed that solitary and gregarious individuals from different populations representing the two mtDNA clades had almost non-overlapping distributions based on multiple song and morphological measurements. Carrier frequency differed between the two, and gregarious males were more reluctant to sing overall. Mirror size predicted carrier frequency; however, the relationship between mirror size and surface area varied between solitary and gregarious forms, suggesting that factors above and beyond mirror size contribute to carrier frequency.ConclusionThe two clades of Mormon crickets differ in a broad suite of independent traits that probably justify subspecies status (the two can successfully mate so may not be reproductively isolated). However, our results emphasize the importance of carefully distinguishing the ultimate causation of differences between traits used to delineate species or subspecies boundaries.

Highlights

In acoustic species, traits such as male calling song are likely to diverge quickly between allopatric populations due to sexual selection, and divergence in parameters such as carrier frequency, chirp structure, and other important song characters can influence sexual isolation
In a multivariate discriminant analysis, cross-validation successfully classified all samples into the correct form, and the traits with the highest loadings on the discriminant function were: head capsule width (HCW), elytral surface area and mirror surface area, and carrier frequency, chirp rate and peg strike rate
The behavioural and morphological differences we have found between solitary and gregarious Mormon cricket populations are consistent with the previous discovery of a genetic division that implies a distinct Pleistocene history

Summary

Introduction

Traits such as male calling song are likely to diverge quickly between allopatric populations due to sexual selection, and divergence in parameters such as carrier frequency, chirp structure, and other important song characters can influence sexual isolation. First that the forms differ in song and second that gregarious males are more reluctant to sing than solitary males due to sex role reversal. Selected traits – such as male calling song – have been suggested to be susceptible to divergence in allopatrically isolated populations [1], and sexual selection-induced differences in song have been proposed to be significant drivers of speciation [2,3,4,5,6]. Gryllidae, oscillation of the mirror (or harp in gryllids) is not damped between peg strikes and calls are "musical", or pure tone. Under the "clockwork cricket" model of stridulation developed from studies of Gryllus campestris [15,16], the dominant or carrier frequency of male song is determined by properties of the harp, and in tettigonids, the length of the mirror frame has been suggested to determine carrier frequency [17]

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