Highlighting the article written by T. Takeuchi, R. Takahashi, T. Kyoshi, M. Nakamura, Y. Minoshima, J.-I. Takahashi in Insectes Sociaux
Written by Insectes Sociaux Editor-in-Chief, Michael Breed
In this issue of Insectes Sociaux, Takeuchi and colleagues reveal the genetic origins of an invasive population of Vespa velutina (the yellow-legged hornet) in the western islands of Japan. This invasive wasp is a predator on the honey bee species Apis cerana in its native range and can prey on Apis mellifera outside of its normal range (Monceau et al 2013). Native to the southern part of Asia and to Indonesia, it has also been introduced into Korea and Europe. They use mitochondrial DNA sequences to generate a cladogram for populations of this wasp and are also able to draw conclusions about genetic variation in the invasive population in Japan. Genetic variation may support the phenotypic flexibility exhibited by some invasives and consequently it is an important feature to characterize in invasive populations.
The role of social insects, such as this hornet, as biologically invasive species is well known, principally because of the prominence of invasive ants through the last 100 years in ecological studies and their creation of important issues in public health and agriculture. The numerous exemplars of high-impact invasive ants include the red imported fire ant, Solenopsis invicta, the pharaoh ant, Monomorium pharaonis and the Argentine ant, Linepithema humile. The Formosan termite, Coptotermes formosanus, is a major pest in many habitats. The western honeybee, Apis mellifera, is invasive throughout the Americas, first as an introduction by European colonialists on the east coast of North America in the 1600s, and then by the introduction of a more aggressive form in the 1950s in Brazil.
In some cases invasive social insects, such as the fire ant and the Argentine ant, are perceived to have essentially overrun an entire ecosystem. These species cause massive shifts in the terrestrial invertebrate fauna, impact the reproductive success of ground nesting birds, and have ripple effects on other trophic levels. However, many social insect invasions occur quietly and go largely unnoticed because the ecological impacts of the invasion are subtle and there are no apparent public health or agricultural implications of the invasion. Whether the impacts are large or small, understanding the processes of biological invasions is a key question in evolution, ecology and behavior.
What makes some social insects such effective invasive species? Ecological flexibility, high reproductive rates, and ability to disperse within the landscape all must be important factors. But many populations of invasive species, social insect or otherwise, survive very narrow genetic bottlenecks. Introductions of species to new areas often involve the transport (typically by humans) of a very small number of individuals or colonies. This may translate into invasive populations with low genetic diversity.
Takeuchi et al (2017) sequenced three mitochondrial genes, COI, Cytb, and 16S rRNA, from samples of V. velutina collected across its natural range, plus invasive populations in Japan and Korea. Their results show that this species is likely monophyletic, but that there are two relatively distinct geographical clades, one in Indonesia and Malaysia, the other more broadly distributed in continental Asia. The invasive Korean population nests within the populations from China, and the invasive Japanese population probably derives from the geographically nearby Korean population.
Significantly, Takeuchi et al (2017) found no genetic variation in these genes in the Japanese population. While their finding is unusually low, even for an invasive that has gone through a genetic bottleneck, it is by no means out of the ordinary to observe invasive social insect populations that derive from a few individuals or a few colonies. In the case of V. velutina, Takeuchi et al. (2017) argue that fertilized queens could easily be carried along in goods transported by humans and that, in a practical approach to regulation of invasions, vigilance for fertilized queens might be useful. It would be very interesting to compare the social flexibility and ecological adaptability of the Japanese population of V. velutina with Chinese populations to see if these features have been affected by the bottleneck.
These findings raise the very interesting question of how invasive social insects are able to retain ecological and social flexibility, keys to being successful invasive species, through periods of small population size. Mechanisms for carrying characteristics that are key to invasiveness through a bottleneck could include having multiple queens in colonies mating more than once, plasticity in phenotypic expression, or rapid evolution of genetic diversity via mutation. Each of these strategies could be effective, and future work building on approaches like those of Takeuchi et al (2017) should help to explain the properties that make some social insects such effective invasive species.
Monceau K, Arca M, Leprêtre L, Mougel F, Bonnard O, Silvain J-F, et al. (2013) Native Prey and Invasive Predator Patterns of Foraging Activity: The Case of the Yellow-Legged Hornet Predation at European Honeybee Hives. PLoS ONE 8(6): e66492. https://doi.org/10.1371/journal.pone.0066492
Takeuchi T, Takahashi R, Kyoshi T, Nakamura M, Minoshima Y, Takahashi J-I. (2017) The origin and genetic diversity of the yellow-legged hornet, Vespa velutina introduced in Japan. Insect Soc DOI: 10.1007/s00040-017-0545-z