Biological invasions are a major threat to biodiversity and the dynamics behind successful invasions remain largely unresolved. Invasion genomics can be used to shed light on genetic diversity pattern during invasion events and to reconstruct the demographic history of invasion events.
Biological invasions have attracted much attention in recent years given their major threat to biodiversity and ecosystems worldwide. Human aided transport of species can have devastating impacts on ecosystems and is regarded as a major driver behind the ‘homogenization’ of the worlds´ biota, especially for marine systems. “But what characterizes a successful invasion event? And how does genetic diversity translate into invasion success?” asks Cornelia Jaspers, first author of a study from the Centre for Ocean Life at the Technical University of Denmark, DTU Aqua and the centre leader of the Villum funded Centre for Gelatinous Plankton Ecology & Evolution, which was published in PNAS this week - Jaspers et al. (2021). The international team from Denmark, Germany, France, USA and Bulgaria addresses these questions in one of the most successful marine invasive species world-wide, the comb jelly Mnemiopsis leidyi, by sequencing the entire genome of multiple native and invasive populations. Native to the east coast of Americas, Mnemiopsis was first sighted in western Eurasia in the 1980´s and has since invaded large areas of Western Eurasia. Irrespective of its large impact, specific invasion dynamics and propagule loads during introduction events remain so far unknown, as common for most non-indigenous marine species, where introduction events remain elusive.
By whole genome resequencing of 72 individuals for different native and invasive populations the authors could evaluate genetic differentiation and genetic diversity during the different invasions and use state-of-the-art coalescent methods to reconstruct the invasion routes and demographic history of the invasions. The authors find different invasion histories for each of the distinct Mnemiopsis invasion events into western Eurasian waters. These included cases of similar, increased and decreased genetic diversity compared to the native populations in the NW Atlantic. “It seemed that after all, the genetic diversity did not mater too much, since all non-indigenous populations were similarly successful and resulted in considerable ecosystem impact in the non-native habitats”, says Marti Pujolar, co-author of the study from the Centre for Gelatinous Plankton Ecology & Evolution at DTU Aqua, Denmark. This illustrates that genetic diversity, per se, is not necessarily the driving force behind invasion success, and refutes one long-standing hypothesis in invasion biology, the genetic paradox of invasions. By resolving temporal dynamics of invasion events, this study uncovered hot-spots of ongoing, recurrent species introductions. “Specifically, for the invasion of Mnemiopsis leidyi in Northern Europe, we find low genetic differentiation coupled with an increase in nucleotide diversity, which suggests multiple transfers of a few individuals over an extended but recent time period from slightly different areas within the native northern range,” explains Cornelia Jaspers. In contrast, the southern invasion into the Black Sea shows a high population differentiation between native and invasive populations, but is characterized by similar levels of genetic diversity. Demographic reconstruction indicates that the invasion into the Black Sea was of a much larger magnitude compared to the Northern invasion, characterized by an additional rapid population expansion shortly after its establishment, which can be attributed to the initial lack of predators. However, the current lack of gene flow and continued divergence between the southern native and invasive populations has shaped the high population differentiation observed to date, possibly explained by a change in shipping routes of oil tankers after the collapse of the Soviet Union.
With regard to the most recent colonization of the Western Mediterranean, the authors could re-construct a stepping stone introduction from the Black Sea. This secondary range expansion is characterized by a significant reduction in genetic diversity. However, even although a severe genetic bottlenecks was detected during the early invasion event, which is the common case for most invasive species, this had no detectable negative effects for later invasion success. The fact that the authors show all cases of genetic diversity within the same invasive species over different invasion events indicate that genetic diversity alone is not the major contributor to invasion success of a species. Instead, resolving temporal invasion dynamics via whole genome re-sequencing data are important for understanding invasion events. “Despite international conventions in place to halt species introductions, our data suggest that the North Sea is subject to contemporary and recurrent Mnemiopsis introductions. Uncovering this temporal context is essential to understand current invasion risk of areas and halt species introductions in the long run,” concludes Cornelia Jaspers.