Genetic diversity, host-specificity and unusual phylogeography of a cryptic, host-associated species complex of gall-inducing scale insects

1. Australia has a unique and speciose gall-inducing scale insect fauna that is primarily associated with Myrtaceae. Much of the diversity is currently undescribed or uncharacterised. 2. This study concerns Apiomorpha munita (Hemiptera), a scale insect that induces characteristic four-horned galls on eucalypts of subgenus Symphyomyrtus and exhibits extraordinary karyotypic diversity (2n = 6 - 2n > 100). The three described subspecies of A. munita are each confined to hosts in different sections of Eucalyptus. Previous chromosomal data, however, cast doubt on the validity of the groupings, as two of the subspecies share multiple, different karyotypes (2n = 6, 20, 22, and 24). 3. Allozyme data were used to examine species delimitation, chromosome evolution, host associations and population structure in A. munita. 4. A cryptic-species radiation was revealed, with at least five taxa each restricted to a discrete set of host eucalypt species. This is consistent with host-associated speciation. 5. Karyotypic variation within A. munita partially fits the five distinct genetic groups, but there are additional chromosomal changes that are not accompanied by detected genetic differentiation. 6. The population structure of taxa within the A. munita species complex suggests that there are high levels of inbreeding, as would be expected for scale insects in which adult females are sessile. Some genotypes, however, are found over great distances (up to 1100 km). This is an unusual population structure because it combines low mobility and local differentiation with occasional long-distance dispersal, probably mediated by wind-dispersal of first-instar nymphs.