Demography and molecular ecology of the solitary halictid Lasioglossum zonulum: with observations on Lasioglossum leucozonium

Abstract

Halictid bees are excellent models for questions of both evolutionary biology and molecular ecology. While the majority of Halictid species are solitary and many are native to North America, neither solitary nor native bees have been extensively studied in terms of their population genetics. This thesis studies the social behaviour, demographic patterns and molecular ecology of the solitary Holarctic sweat bee Lasioglossum zonulum, with comparisons to its well-studied sister species Lasioglossum leucozonium. I show that L. zonulum is bivoltine in the Niagara region of southern Ontario but is univoltine in a more northern region of southern Alberta. Measurements of size, wear and ovarian development of collected females revealed that Brood 1 offspring are not altruistic workers and L. zonulum is solitary. A large proportion of foundresses were also found foraging with well-developed ovaries along with their daughters, meaning L. zonulum is solitary and partially-bivoltine in the Niagara region. L. zonulum being solitary and univoltine in Calgary suggests that it is a demographically polymorphic and not socially polymorphic. Thus, L. zonulum represents a transitional evolutionary state between solitary and eusocial behaviour in bees. I demonstrate that Lasioglossum zonulum was introduced to North America at least once from Europe in the last 500 years, with multiple introductions probable. Most North American specimens share the same mitochondrial DNA haplotype as those in Europe, with a small portion from western North America possessing distinct sequences. Investigations using microsatellite markers found North American populations to have a deficit of heterozygosity, and Bayesian analysis suggests that there are 3-4 lineages of L. zonulum in North. It is theorized that introductions could also be from Europe, Asia, or could even represent a native population which arrived via the Bering Land Bridge. I suggest that the plasticity found in L. zonulum may have a genetic cause and exists in North America due to the multiple introductions and potentially diverse geographic origins of this species. The outcome of my studies highlight why Lasioglossum zonulum is a model organism for the study of how eusociality evolved and why it warrants further and more in-depth study.