Systematics, Phylogeny And Acoustic Evolution In Field Crickets (Orthoptera, Grylloidea, Gryllinae)

Abstract

Since several decades, field crickets of the subfamily Gryllinae under order Orthoptera have been used as a model group by researchers working in the field of neuroethology, behavioural ecology and bioacoustics. Subfamily Gryllinae is a much diversified group and members are diverse in their biological traits however the most commonly used model systems in the research fields belong to only three genera: Gryllus, Teleogryllus and Acheta. All the three genera are related to each other therefore, they fail to represent the wide diversity of ecological and behavioral traits that are encompassed by other members of Gryllinae. To understand the diversity in an evolutionary framework in field crickets, it is thus necessary to take into account the evolutionary background of the model systems. A phylogenetic analysis is a prerequisite for all evolutionary studies. A phylogeny hypothesizes the relationship among the studied taxa on the basis of homology drawn from all the characters that were used for the analysis. The phylogenetic tree is a branching diagram where related taxa are grouped together at nodes and the terminal ends of the branches are the studied taxa. The characters are mapped on to this tree thus indicating the hypothetical ancestral state of each character at the nodes and explaining the transformational stages of each character that gave rise to the current character state. This procedure helps in understanding the evolutionary history and understanding the patterns of relationships among taxa that in turn also help in identification of species. This process of identification and classification by phylogenetic reconstruction is formally known as “phylogenetic systematics”. The main goal of this thesis work was to understand the phylogenetic relationships among field crickets of the subfamily Gryllinae. This study is of importance in the field of orthopteran classification because since the 19th century, taxonomists proposed several taxonomic rankings and various classificatory schemes either by describing new classificatory levels or by upgrading existing taxonomic levels to higher levels. In addition, different sets of morphological characters have been used in proposing the classification hypotheses which in turn leads to the existence of several mutually exclusive hypotheses. Although there have been several changes in the taxonomic positioning of these crickets under Gryllinae, none so far have been based on a phylogenetic hypothesis. Therefore, I described 100 characters based on external and internal morphological structures of crickets belonging to 17 genera to test all the previously proposed classificatory hypotheses, provide a scheme for classifying these crickets under different hierarchical levels under Gryllinae and a strong base for conducting further evolutionary studies. The results lend full support to one of the previously proposed hypotheses for classification. Subfamily Gryllinae is a monophyletic clade where all the field crickets should be grouped under two tribes. The results also invalidate several tribes that have been erected in recent years. The second aim was to test species concepts by examining concordance in species boundaries generated from different data sets and to study the pattern of song evolution in field crickets. Therefore, for this study a field cricket belonging to genus Itaropsis was selected. Only one species of Itaropsis is known from the Indian subcontinent which was described from Sri Lanka. Interestingly, five call types of Itaropsis were encountered when a field survey was performed at four different localities in the Western Ghats of India. Males of the all five call types (varying from long trills to short chirps) looked morphologically very similar to each other. However, they differed in their calling song structure, I described four call features (carrier frqeuncy, syllable period, syllable duration and call duration) and classified the individuals into clusters by calculating distances using these call features. The clusters thus obtained were considered to reflect different species under the genus Itaropsis. To assure that the clusters thus obtained reflect different species, concordance between the results was tested by comparing these results with the clades of an available phylogeny for the collected individuals of this genus using both morphological and molecular data. The acoustic clusters showed concordance with the phylogenetic lineages largely however an absolute match was not found suggesting that these lineages cannot be called as distinct species. The pattern of evolution of songs in Itaropsis was examined by optimizing the call features on the phylogenetic tree derived from molecular data. This showed short chirps with a dominant frequency of 7 kHz to be the ancestral call type within this genus, from which long trills with long syllable durations have evolved. There were no significant patterns in the evolution of carrier frequency. In most crickets, the calling songs that are produced by males to attract their conspecific females are considered species specific therefore, the calling songs serve as an additional tool in identification of crickets. The third aim of my work was to use only call features of crickets in delineation of species but for higher number of species. Therefore, for this I used recorded calling songs of 14 species of field crickets and described their call features. I then used either five or seven call features to construct clusters based on Euclidean distances between the songs of all pairs of individuals. I also carried out a discriminant function analysis of these data to test the efficiency of species identification when taxa were specified a priori. I varied the number of species used for the analysis systematically from 5 to 14 to examine the effect of varying the number of taxa on correct species identification. The results showed that taxa number can be increased upto ten to obtain almost correct classification however, using six to seven taxa is optimal since they provide 90% accuracy in classification. To the best of my knowledge, this study on understanding the phylogenetic relationships among field crickets is the first to be carried out at subfamily level. Description of new call types reflecting multiple or incipient species under the genus Itaropsis and use of acoustic features in species identification of Indian field crickets is also a new contribution to the area of orthopteran systematics and phylogenetics.