During most of the Pleistocene Epoch (about 2.5 million to 10 thousand years ago), almost all of Canada was covered in great sheets of ice. The ice came and went a number of times, but it was a dominant force for hundreds of thousands of years. All this time, however, the land of the far northwest—central Yukon and Alaska—remained ice-free and was joined to eastern Siberia by an isthmus of land as broad as Alaska—the Bering Land Bridge. This vast expanse of tundra and grassy steppes is known to geographers and biologists as Beringia (Figure 1).
Roaming across Beringia were herds of mammoths, antelope and bison, each hunted by lions, wolves, and scimitar cats. The ecosystem was predominantly Asian in origin and completely isolated from the rest of North America by two thousand kilometres of ice. Today, 10 thousand years after the last ice barrier disappeared, most of the big mammals are gone, but many of the plants and insects of Pleistocene Beringia remain. These are Asian species in North America.
Royal BC Museum Research Associates Syd and Rob Cannings study some of the special insects of Beringia and northern British Columbia (BC) and the fascinating connections they make with the Asian fauna. Today some of these species may live on both sides of the North Pacific Ocean and others may be separated by it—their characteristics and present geographical distributions reveal complex evolutionary histories.
The striped emeralds (Somatochlora species) are a dominant group of dragonflies in boreal North America. In general, they are distinguished from other dragonflies in the region by their dark, metallic bodies and their stunningly green eyes. In the far northwest, the Treeline Emerald (S. sahlbergi) (Figure 2) reaches into northwestern Canada from its largely Siberian base; it is the only dragonfly restricted to Beringia in North America. Where its range overlaps with that of the Ringed Emerald (S. albicincta) and the Hudsonian Emerald (S. hudsonica), (Figure 3), considerable hybridization occurs. Hybrids have been found in central Alaska, the Mackenzie River Valley, and the northern Yukon. Hybrids have colour patterns and structural characteristics intermediate between those of the parent species.
Obvious hybrids are not often encountered in dragonfly study, and this is the result of ‘good’ female choice. Male dragonflies are not very selective when seeking mates; they will try to mate with any female that is about the right size and shape! It is the female that decides whether or not to accept the male. So it seems that hybrids result when females make a mistake by accepting a male from a different species.
Normally, the three species of emeralds separate themselves by larval habitat: the Treeline Emerald develops in permafrost ponds with floating moss (Figures 4, 5), the Ringed Emerald prefers shallow ponds with little vegetation, and the Hudsonian Emerald lives in ponds with tall sedge edges. But sometimes, wetlands occur that combine two or more of these habitats in close proximity, and in these places the three species may encounter one another.
Hybrids (as identified by structural features) make up about 20% of individuals captured in the areas where hybrids have been found. The hybrid indices (Figures 6) are based on a number of characteristics scored for each individual, including face colour, thorax and abdominal patterns, leg length and total length.
To study this hybridization in greater depth, we sequenced the mitochondrial CO1 gene from many specimens of Somatochlora. Mitochondria are cellular structures that, among other things, produce most of a cell’s energy. Because mitochondrial genes are inherited only from the mother, they record the individual’s maternal lineage very well but tell us nothing about paternal ancestors.
When arranged by their CO1 gene sequences (Figure 7), the Treeline Emerald group (violet highlight), although well-defined, has a number of individuals of what appear to be White-ringed Emeralds within it. These individuals are evidence of genetic introgression between the two species. Introgression is the movement of genes from one species into the gene pool of another by the repeated mating of hybrid individuals with one or both of the parent species. Because the mitochondria are inherited from the offspring’s mother, these individuals are apparently the result of hybrid matings between male White-ringed Emeralds and female Treeline Emeralds. However, some or all of these hybrid matings could have occurred many generations ago, and the individuals are now relatively ‘pure’ White-ringed Emerald that carry the original Treeline Emerald mitochondria in their maternal lineage.
Other genetic evidence tells us that the Treeline Emerald is an old, well-established species in Asia; the evidence of hybridization here tells us that it is probably a new immigrant to North America. Female Treeline Emeralds apparently still have difficulty recognizing Ringed and Hudsonian Emerald males as different from those of their own, and can mate with them in error. Similarly, female Ringed and Hudsonian Emeralds apparently make the same mistake with Treeline Emerald males.
Despite our earlier collections of apparent hybrids between Hudsonian and Treeline Emeralds, we didn’t detect any evidence of introgression between those species in the genetic analysis (Figure 7). This is probably because we had very few recent specimens of Hudsonian Emeralds from Beringia with which to work. Unexpectedly, evidence of genetic introgression between Hudsonian and White-ringed Emeralds appears in the cluster within the Hudsonian group (yellow highlight); these specimens are from BC’s north coast. There also appears to be introgression in other groups as well. Why apparent hybrids between White-ringed and Treeline Emeralds (top, green highlight) cluster with Whitehouse’s Emeralds is unknown.
Lasiopogon robber flies
Rob introduced some of his research into Lasiopogon robber flies (Figure 8) in the Spring 2014 issue of Curious, much of which is included here. Many of the Lasiopogon flies now inhabiting eastern Asia probably evolved from populations living in eastern North America as the mixed forests of the northern latitudes cooled and shrank significantly before the middle of the Miocene Epoch, about 15 million years ago. Many groups of organisms, including many insects and plants, show this biogeographic pattern. In eastern Asia Lasiopogon evolved into several groups, including an important part of the Beringian robber fly fauna. Three Beringian species (L. hinei, L. canus, and L. prima) are closely related to Eurasian flies —they or their ancestors probably returned to North America through ice-free Beringia. These flies live on the south-facing grassland slopes (Figure 9), in dune habitats (Figure 10), and along streams (Figure 11) in the far northwest of North America. Similar habitats in Far-eastern Russia are shown in Figures 12 and 13.
Lasiopogon hinei ranges mostly across Eurasia (Figure 14); there, it was first named L. sibiricus, but Rob’s investigations showed that it is the same species as L. hinei, described from Alaska much earlier. It certainly lived in Beringia when Far-eastern Russia and northwest North America were connected. Part of the population was isolated in North America after the land bridge was last flooded by rising sea-levels and, since the disappearance of continental glaciers, it has extended its range southward along the eastern flanks of the Rockies of BC into central Alberta. Specimens from the islands of northern Japan and Sakhalin are darker and more bristly than the mainland flies. At first glance, they look like a different species, but detailed examination of the genitalia proves that they are the same — the changes in their outward appearance are the result of several thousands of years of geographic isolation of these islands from mainland Asia.
Lasiopogon prima has a similar distribution in North America to L. hinei. Since the last glaciation, from northern BC it has expanded its range to the southeast into Alberta and as far east as the Athabaska dunes of Saskatchewan. In Alaska and Yukon, it has followed tree-covered river valleys to the Arctic coast. It is the North American representative of a group of three species that is now divided by the Bering Strait. In Eurasia, from Finland east to the Kolyma River basin, its close relative, L. septentrionalis, is widespread. The other close relative, an undescribed fly discovered by RBCM Research Associate Tristan McKnight, lives along rivers in the grasslands of Mongolia (Figure 15). These species are inhabitants of northern forests and grasslands, and sister species in East and West Beringia represent a pattern that results from the separation of forest habitats in Beringia after climatic cooling in the Pliocene (about 2.5 to 5.3 million years ago) or by the first appearance of the Bering Strait itself in the Pliocene about 2.5 million years ago.
The most common Beringian Lasiopogon, L. canus, is not known yet from BC, despite much searching. It is restricted to Alaska, Yukon and the northwestern Northwest Territories, a distribution typical of a species that spent the last glaciation in eastern Beringia, then dispersed only short distances south and east after the ice melted. I am now studying and describing its closest relative, a newly discovered Lasiopogon robber fly (Figure 8) living along rivers in central Alberta and the Peace River region of BC. Probably the ice sheets divided the original ancestral population and allowed differentiation of the two species present today. The undescribed species might have evolved in an unglaciated part of the region, or it could have spent the glacial maximum south of the ice and moved northwards following retreat of the glaciers.
Dune Tachinid Fly
The Dune Tachinid Fly, Germaria angustata, is a modest-sized, bristly fly (Figure 16) that is found across the Old World, but is probably most common in dunes of Mongolia and China (Figure 17). It is also found in the coastal dunes of Europe, but is rare there. Tachinid flies are parasites of the larvae of other insects, often moth larvae. The host of the Dune Tachinid Fly remains unknown, but may be the larva of a cutworm moth that lives underground during the day and comes to the surface at night to feed on the base of the grass. Female Dune Tachinid Flies fly low over the open sand, alighting on single stems of grass, walking to the base of each, and apparently depositing an egg there.
In Canada, this fly was first discovered in the dunes at Carcross, Yukon (Figure 10), in 1980 by Monty Wood, an entomologist at the Canadian National Collection of Insects in Ottawa. At first, Monty thought that he had discovered a new species, but soon realized that this was an Asian fly that had entered Beringia. He subsequently searched for it in the other large dune systems of northwestern North America—the Nogahabara Dunes of western Alaska, and the Athabasca Dunes and the Great Sand Hills of Saskatchewan—all to no avail. It seemed that it was indeed restricted in North America to the Carcross dunes.
Because it was apparently restricted to only one locality in North America, the Dune Tachinid Fly caught the attention of the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). In an effort to find out just how rare it really was, Syd searched for this fly throughout the southwestern Yukon from 2008-2010, and found that it was more widespread than previously thought. It is now known from 14 dunes, from Kluane National Park in the west to Whitehorse in the east. Still, this isn’t a large area and the dunes make up a limited habitat; in 2011 the Dune Tachinid Fly was assessed as a species of Special Concern by COSEWIC.
Over the past decade or more, Syd has organized annual field collections in Yukon and northern BC as part of this and other work on species at risk out of the Whitehorse office of the Canadian Wildlife Service, Environment Canada. Most of these collections are deposited in the RBCM entomology collection and are often databased with federal funds. Such collections are invaluable in determining the distribution and conservation status of rare species in northern BC and Yukon. They provide unusual insight into the evolutionary history and biogeography of related insects across both the eastern and western Pacific worlds.