Dissertation Research: Heterochrony in Insects: Phylogeny and Evolution of Acoustic Characters in the True Crickets (Orthoptera: Grylloidea)

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0073187Shaw and Jost Crickets have always interested biologists for their calling songs used incourtship. In most species, male crickets produce a song which is heard and recognized by potential female mates. There are thousands of cricket species worldwide, each of which has a unique type of song. Previous studies have looked for ecological or adaptive reasons that cricket calling songs are so diverse. However, few evolutionary studies have looked at cricket song from a morphological perspective, by examining the structures which produce and receive the sounds. In crickets, these acoustic structures are specialized front wings in males, and hearing organs on the front legs of both sexes. These structures are extremely diverse in crickets (there are ca. 3,500 described species in the world), but in many species are completely absent. Developmental studies have found that acoustic body parts are strictly adult structures which do not appear until the final life stage. In addition, many other structures -- such as hind flight wings, eyes, antennae, and genitalia -- are often reduced or absent in non-acoustic cricket species. The simplest way that many adult traits could be reduced or absent in a species is by changing the timing of overall growth. In other words, many non-acoustic cricket species may simply be morphologically immature forms of closely related acoustic species. This project will test the hypothesis that morphological diversity in acoustic structures can be explained by changes in the timing of growth and metamorphosis, a process known as heterochrony. Graduate student Manda Jost, with guidance from faculty adviser Dr. Kerry Shaw, is studying the phylogenetic relationships of exemplar species of crickets and their acoustic evolution. The first step to testing the heterochrony hypothesis will be a broad, detailed survey of cricket morphology for hundreds of species. A diverse sample of species will then be selected from regions around the world, and a phylogeny will be produced using DNA sequence from the mitochondrial gene CO-I. Statistical and comparative tests will then determine whether morphological "maturity" of acoustic structures is significantly correlated with the "maturity" of other morphological structures which are functionally unrelated. Demonstrating that many functionally unrelated traits show simultaneous change would support the hypothesis that these traits are all under control of a basic growth schedule. This idea is not an alternative to ecological or adaptive hypotheses, but provides a physical mechanism which could produce the diversity we observe. This study will result in the most detailed phylogeny to date of the crickets, while using new ideas and methods to study the diversity of cricket acoustic characters.