Introgressive hybridisation and incipient ecological speciation amongst saltmarsh Aphrodes leafhoppers

Ecological and host adapted races provide evidence that evolutionary divergence and sympatric speciation can occur through divergent natural selection in the face of continued gene flow. Likewise, hybridisation and introgression (interspecific gene flow) are commonly identified in natural populations, between what are described as distinct taxa. These processes have implications for how we define species and the processes necessary for the persistence and initiation of species and speciation, above and below the species level. The main focus of the present study was elucidation of the nature and extent of differentiation, and processes involved in shaping diversity within and between, species of the Aphrodes leafhopper genus, Curtis 1833, particularly from UK saltmarshes. A multidisciplinary approach was taken, combining the use of morphological, behavioural (vibrational mating signals), mitochondrial DNA (cytochrome oxidase subunit I gene sequencing) and multiple genome-wide nuclear marker (amplified fragment length polymorphism) analyses to test hypotheses relating to taxonomy, ecological speciation and hybridisation among Aphrodes leafhoppers. Of primary interest were: 1) identification of Aphrodes inhabiting saltmarshes, and first confirmation that two species (A. makarovi and A. aestuarina) exist there; 2) comparison of divergent ecological lineages of inland and estuarine A. makarovi, showing possible incipient speciation and evidence of convergent morphological evolution of estuarine A. makarovi and A. aestuarina; 3) exploration of the evolutionary significance of an introgressed hybrid population of A. aestuarina, found only in the Medway estuary, showing complete mitochondrial capture and some nuclear introgression