Can invasive plants become new species in their invaded range?

Ramona Irimia and Daniel Montesinos discuss their recent Journal of Ecology article: “Experimental admixture among geographically disjunct populations of an invasive plant yields a global mosaic of reproductive incompatibility and heterosis. Find out more about their insights into the biogeographic patterns of variation in reproductive success in the yellow star-thistle, an invasive weed.

Invasive plants are interesting model systems because after they are introduced into new non-native regions there is usually little and only occasional gene flow between populations in the native and non-native regions. As a result, native and non-native populations separated by geographical barriers are de facto allopatric or, perhaps more accurately, neo-allopatric (Montesinos et al. 2012).

The general theory about allopatric evolution states that populations in allopatry develop local adaptations and that, given enough time, reproductive barriers would gradually arise. This process is fundamental to the theory of allopatric speciation. However, we have very few opportunities to study allopatric speciation processes directly, as we might not know when the process started, and these processes tend to take centuries, if not millennia. Or so we though.

As it happens, neo-allopatric invasive plants are ideal systems to study the early stages of allopatric speciation, and specifically to study how reproductive systems respond to isolation in allopatry. Irimia and co-authors (2021) just published a pioneer study assessing just that for an invasive thistle native from Europe and invasive in the Americas and Australia. Seeds were collected from six different countries in both the native (Turkey, Spain) and non-native (California-USA, Chile, Argentina, Australia) ranges. Plants were grown to flowering, and crosses within and across world regions were done. The result is a global mosaic of increased and decreased reproductive outputs.

The yellow star-thistle, Centaurea solstitialis, is globally distributed herbaceous annual plant species. Photo: Eugene Zelenko (CC BY-SA 4.0)

Invasive populations in the Americas generally produced less seeds when pollinated by plants from any other world region. This suggests that these plants are well-adapted to their invasive range, and that in only slightly over 100 years/generations they might have already developed some reproductive barriers “protecting” them from genes from other regions that might not be as well-adapted to the local conditions. This process is known as “reinforcement” and serves the purpose to fixate well-adapted genotypes in the populations, and to protect them against potentially maladapted genes from other imported genotypes. In fact, a previous study confirmed just that, by assessing the competitive ability of inter-regional hybrids and finding that hybrids between highly competitive plants from California and poorer competitor plants from Spain presented intermediate competitive ability. In other words, if genotypes from Spain were introduced into California today, the resulting hybrids would be poorer competitors than the current Californian genotypes (Montesinos and Callaway, 2017).

Contrarily to populations in the Americas, non‐native populations from Australia, as well as individuals from the native range of Spain, demonstrated an increase in reproductive outputs when exposed to pollen from other world regions. This suggests that plants in these regions might not present large genetic variability and that they present inbreeding. As such, imported genes can have a very “positive” effect on these populations, increasing their vigour and giving them the genetic diversity that they might need for selection to act upon before they can become more successful.

Overall, the differences found showed an asymmetrical response to inter‐regional gene flow, but there was no overall evidence of direct isolation by distance. The speed of adaptation and the accumulation of reproductive incompatibilities among allopatric populations of invasive species might be more rapid than previously assumed, and the global mosaic of reproductive outputs found showcases an array of evolutionary processes unfolding during colonization at large biogeographical scales. This knowledge can prove useful in understanding historical allopatric speciation processes for other non-invasive species by using invasive species as “real time” model study systems.

Ramona Irimia University of Coimbra, Portugal & University of Tübingen, Germany

Daniel Montesinos James Cook University, Australia & University of Coimbra, Portugal

Read the full article: Experimental admixture among geographically disjunct populations of an invasive plant yields a global mosaic of reproductive incompatibility and heterosis

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