Abstract
Elucidating the historical processes that are responsible for the contemporary geographic distributions of evolutionary lineages is the major goal of phylogeography. Here I assess phylogenetic and biogeographic relationships within stoneflies (order: Plecoptera) using mitochondrial (COI) and nuclear (18S; H3) DNA. Sequence data were obtained from almost 500 southern hemisphere plecopteran specimens, with Bayesian phylogenetic analyses performed to elucidate the relationships among lineages. A variety of analyses were used to characterise the biogeographic forces responsible for the contemporary distribution of these lineages, and to examine how flight loss impacts stonefly evolution and biogeography.
Geological data are used to calibrate a multilocus phylogeny of stoneflies. The two stonefly sub-orders, Arctoperlaria (northern hemisphere) and Antarctoperlaria (southern hemisphere), are estimated to have diverged during the Jurassic, consistent with their vicariant formation driven by the breakup of Pangaea. Subsequently, a single Antarctoperlaria lineage apparently dispersed into the southern hemisphere during the Cretaceous. The disjunct distribution of southern hemisphere lineages is best explained by combination of both vicariance and dispersal, with most of the divergences between South American and Australian lineages consistent with vicariance, whereas dispersal best explains the origin of South African and New Zealand lineages.
Comparative phylogeographic analysis of six alpine stonefly genera was used to test a vicariant evolutionary hypothesis for the origin of the ‘biotic gap’ in the central South Island, New Zealand. The magnitude of north-south genetic differentiation was strikingly similar across all six genera (ranging from 0.074-0.091), with a test for simultaneous vicariance confirming that divergence is consistent with a single evolutionary event. The concordant cladogenesis detected across multiple taxa was consistent with vicariant isolation caused by the onset of glaciation in the late Pliocene. This indicated an important cladogenetic role for glaciation, an abiotic evolutionary process that is more typically associated with loss of biodiversity.
The genetic and biogeographic impact of wing reduction on New Zealand stoneflies was assessed via comparative phylogeographic analyses (COI; H3) of strong-flying Zelandoperla decorata versus the co-distributed but weak-flying Zelandoperla fenestrata species group. Consistent with its capacity for strong flight-mediated dispersal, Z. decorata exhibited no substantial phylogeographic differentiation across its broad South Island range, while conversely the weak-flying fenestrata species group exhibited substantial genetic structure across both fine and broad geographic scales. Intriguingly, the variable degrees of wing development observed within the fenestrata species group had no apparent impact on levels of phylogeographic structure, which were high regardless of morphotype, suggesting that even fully-winged specimens of this group do not fly. This finding implies that Zelandoperla flight-loss occurs independently of wing-loss, maybe reflecting underlying flight-muscle reduction.
This phylogeographic research sheds new light on the relative importance of dispersal and vicariant events in shaping the contemporary distribution of stonefly lineages in the southern hemisphere.