Discussion
Our genomic study of 50 global populations of a worldwide, selfing weed
revealed several important findings. First, populations were found to
form six unique genotype groups or ecotypes showing very little sign of
admixture and are therefore considered reproductively isolated. Second,
three of the most prevalent of these ecotypes present in Europe are
found most commonly throughout the introduced ranges, indicating that
more than one aggressive genotype was introduced and established. Third,
the multiple independent introduction events we detected and the
distribution of ecotypes have links to colonial history. Finally, the
global distribution of ecotypes was found to be restricted based on
latitude (and therefore environmental factors related to latitude such
as climatic). We therefore propose that multiple successful genotypes
and prior adaptation to climate in the native ranges are likely reasons
for the success of this global weed, such that a the right (prior
adapted) ecotype must have arrived in an introduced location with a
corresponding environment in order to persist and thrive.
Global distribution of ecotypes
Our study is the first to sample populations of Plantago major so
extensively across its global range and to provide an overview of the
global distribution of the species’ genotypic variation. The low genetic
differentiation we found within populations worldwide is in keeping with
what is expected for a highly selfing plant, and with the findings of
past genetic studies of P. major (Mølgaard, 1976; Wolff et al.,
1994; Wolff & Morgan-Richards, 1998; Morgan-Richards & Wolff, 1999;
Barrett, Colautti, and Eckert, 2008). One of our main findings was that
based on genetic distances and shared ancestry tests, individuals from
across the global range cluster into six genotype groups rather than by
population despite extraordinary geographic distances between them.
However, genotypes from a given population were homogenous, such that
you can predict the genotype group of an individual from those of the
other individuals in a given population, based on where in the world it
is growing. A commensal species such as P. major that is known to
flower continuously, to produce wind-borne pollen and thousands of
propagules (seeds) from a single individual, has undoubtedly been moved
around and is still being moved around especially given today’s global
trade networks (McNeill et al, 2011; Early et al., 2016). However, the
lack of admixture or limited evidence of geneflow among genotype groups,
even where populations occur in close proximity, suggests very low rates
of out-crossing (Wolff, 1991), and provides evidence that the groups we
identified are largely reproductively isolated.
The six distinct genotype groups may represent different taxonomic
subspecies or varieties of P. major and/or intermediates between
taxa that may have arisen by historical introgression. Two well-accepted
subspecies, P. major subsp. major and P. majorsubsp. intermedia , have been the focus of past genetic and
ecological studies. For example, Wolff et al. (1994) demonstrated low
levels of polymorphism between populations of both P. majorsubsp. major and P. major subsp. intermedia , but
found notable genetic differentiation between subspecies. Furthermore, a
comparison of Dutch and Scottish populations for the same two subspecies
revealed that genetic divergence between subspecies was stronger than
divergence within populations of the same subspecies between the two
countries (Wolff & Morgan-Richards, 1998). These well-studied
subspecies are sympatric and capable of interbreeding, yet genetic,
ecological, and morphological differentiation between them was found to
be pronounced enough for them to be resolved as separate
species/subspecies likely because high selfing rates limits
interbreeding (Mølgaard, 1976; Morgan-Richards & Wolff, 1999).
Even though our sampling was focused on populations that fit the
morphological descriptions for subsp. major , it is possible that
our genotype groups reflect the complexity of several intraspecific
taxa—or hybrids of them resulting from historical introgression and
being maintained due to reproductive isolation. El-Bakatoushi (2011)
found populations in Egypt to be intermediate between two subspecies,
and that some populations exhibited morphological characters resemblingP. major subsp. major despite evidence of introgression
with P. major subsp. intermedia . Because the goals of our
work were not taxonomic in nature, and because very little is known
about the numbers and global distribution of intraspecific taxa and the
extent of their introgression, we consider the six groups identified in
our study to be ecotypes of P. major sensu lato .
Three of the ecotypes are found to occur in both native and introduced
ranges, and one unique ecotype (from the populations in Yukon and Alaska
[in part]) is found in the introduced ranges. Selfing or clonal
plants have been found to exhibit low genetic variation due to founder
effects and lack of admixture in introduced populations compared to
native populations (Zhang et al, 2010; Ferraro et al., 2015). The three
most prevalent ecotypes we identified in Europe (two in the northern
latitudes, and one in the southern latitudes) are also found to be most
widespread in the introduced ranges; very little genetic differentiation
is seen between native and introduced populations of the same ecotype.
Plantago major is a prime example of a species that increased
drastically in numbers and spread within its native range after the
widespread clearing of land begun during the Neolithic, between
4900–2000 BCE as evidenced by palynological records (Godwin, 1944;
Pysek, 1998; Preston et al., 2004; Kaplan et al., 2016). Weedy and
aggressive ecotypes—which are commonly reported in successful
introduced or invasive plants (i.e. Hawthorn, 1974; Taylor & Keller,
2007; Lambertini et al., 2012)—could have evolved and spread widely in
Europe before being introduced to other continents. We show that not
just one but at least three ecotypes have successfully colonized
introduced ranges and may represent lineages that have evolved increased
abilities to withstand human disturbance and human-altered habitats
(Mølgaard, 1986; El-Bakatoushi et al., 2011; Hufbauer et al., 2012;
Vigueira et al., 2013). The nature of our genomic data and lack of
historical records make it impossible to determine when and where in the
native range these ecotypes of P. major evolved and when their
ranges expanded; however, as is hypothesized for other weedy and
commensal European species, the rapid onset of agriculture and the
profound modification of the native landscape in Europe around the
Neolithic likely facilitated the spread (Godwin, 1944; Hicks, 1971;
Preston et al., 2004, Brun, 2009). Even though the historical herbarium
specimens of P. major were collected up to centuries after it’s
putative introduction to new ranges, DNA analyses of these specimens may
shed light on historical introduction pathways (Dormontt et al., 2007;
Martin et al., 2014).
Multiple introduction events with colonial
ties
Multiple source origins from the native range are inferred for this
global weed. The three most widespread ecotypes found in Europe and the
unique ecotype found in the Middle East (Iran1) were all detected in the
introduced ranges we sampled. Therefore, at least three separate
introduction events occurred from Europe to North America, one event
from the Middle East (Iran) to western North America (California), two
introduction events to the Australasian continent, and at least one
introduction event to each of South America, South Africa, Canary
Islands, Hawaiʻi, Greenland and Iceland.
The introduction of more than one lineage from the native range has been
reported for other introduced species. For example, Guo et al. (2017)
found two introduced lineages in North America for the invasive grassPhragmites australis (Cav.) Trin. ex Steud. – one lineage native
to north-central Europe and Asia occurs in the Great Lakes region of
North America, and another lineage from the Mediterranean region is
naturalized in the Gulf Coast (USA) and in Central and South America.
These patterns conspicuously match patterns we see with ecotypes ofPlantago major from Europe and their distributions in North and
South America.
Unfortunately, the very low genetic differentiation within eachPlantago major ecotype makes it difficult to infer more precise
origins for the introduced populations, despite our extensive sampling
across native and introduced ranges. Furthermore, we cannot rule out
that multiple introduction events of the same ecotype occurred
historically, particularly given the extraordinary dispersal abilities
and commensal nature of the plant. Our data does however support
hypotheses that P. major followed early European colonists to new
ranges (Samuelsen, 2000). The distribution of ecotypes and shared
ancestry inferred between plants in native and introduced ranges reveals
links to early colonial human movements and/or European colonies. For
example, ecotypes found in southern Europe (Spain, Portugal) are also
found in former Spanish or Portuguese colonies in Florida (USA), Peru,
Chile, Brazil, and the Canary Islands, and could have followed colonial
Spanish and/or Portuguese voyagers and settlers between the
15th and 18th centuries (Mancall,
2006). Plants in southern Africa could have been transported and
introduced by Portuguese voyagers along their voyages to southern and
eastern Africa.
Similarly, the two ecotypes prevalent in central and northern Europe and
western Asia likely gave rise to the majority of populations in North
America, as well as Greenland, Iceland, and New Zealand. Plants could
have travelled with early colonial explorers/settlers from France and
England in the 16th and 17thcenturies where these countries had colonial power (Mancall, 2006). In
spite of hypotheses that the Norse played a role in the movement ofPlantago major in their travels across the northern Atlantic
(Samuelsen, 2000), our data do not reveal any specific shared ancestry
between plants in Scandinavia, Greenland and northeastern Canada
(Newfoundland). The earliest known record of the plant in New Zealand is
from 1832; however, the plant likely was introduced well before that
(Webb et al., 1988).
Some populations of Plantago major do not show obvious colonial
ties. For example, although populations from both Melbourne and Perth
(Australia) share ancestry with the southern European populations, there
is no evidence of direct voyages between the Spanish or Portuguese
during early colonial times, although Spanish and Portuguese were both
in southern Africa and southeast Asia, and voyagers from the UK made
stops in South Africa on route to Australia. Plants could have been
introduced to southern Africa and then further dispersed to Australia.
Alternatively, plants could have arrived with the Portuguese to Timor in
the 16th century (Mancall, 2006), approximately 650 km
from the Australian coast, and later dispersed by birds to Australia
(Iwanycki Ahlstrand et al., 2019). The first record for the species in
Australia is in 1770 (GBIF.org), around the same period the English made
their first voyages there, which indicates that plant may have arrived
with earlier explorers such as the Dutch, who arrived in Australia (and
New Zealand) in the 17th century (Mancall, 2006). The
plants we sampled in Hawaiʻi also belong to the southern European
ecotype group. Given that English explorers were the first to reach
Hawaiʻi (Captain James Cook in 1778), after stops in Tahiti and South
Africa, it is also plausible that plant genotypes from more northern
European latitudes also arrived in Australia, South Africa, and Hawaiʻi,
but that the climate or environment did not allow their persistence. DNA
from historical herbarium specimens could help to shed light on this.
The ecotypes found in eastern Asia (South Korea and Japan) were not
found within any of the introduced ranges we sampled. This could be an
artefact of poor sampling across Asia, or could represent a true
biological scenario in which Plantago major ecotypes from eastern
Asia are not as successful in colonizing and spreading in other regions.
At least two ecotypes are recognized in Japan – one restricted to sandy
shores and brackish waters (which is represented in our sampling), the
other considered introduced and weedy (pers. comm. M. Amano, 2020). The
population sampled in Yukon, and some of the individuals from Alaska
(USA) share common ancestry with eastern Asian populations, yet are
found to have a unique lineage. These findings for populations we
sampled in Yukon and Alaska (in part) provide support to the hypothesis
that the northerly latitudes in North America consist of native
populations of P. major that predate the arrival of early
Europeans (Hawthorn, 1974). Increasing sampling and further
investigating of the link between Asian plants and those in northernmost
North America is needed to unravel relationships and migration patterns,
and clarify any possible links between the movement of eastern Asian
plants to northern USA and Canada (for example by Russian colonists or
by earlier human migrations from Beringia [Erlandson & Braje,
2011]).
One unexpected finding was that the population sampled in California was
found to differ from plants elsewhere in North America, and shared
ancestry with the population from Iran1. However, other weedy species
with native distributions in the Middle East have been introduced to and
have become well established in California (i.e. Ortiz et al., 2008;
Meyers & Liston, 2008). The relationships among such populations is
most likely due to similarities in climate. Pathways of migration
between these biogeographic regions remain poorly studied but the
dispersal of weedy species could be linked to the movement of plants for
horticultural trade with which weeds are also moved (Chapman et al.,
2017; Dullinger et al., 2017). We also cannot rule out non-anthropogenic
introduction events, particularly since the genus Plantago is
well adapted for long-distance dispersal (Meyers & Liston, 2008;
Iwanycki Ahlstrand et al., 2019). In a study by Morgan-Richards & Wolff
(1999), populations sampled in Los Angeles (California, USA) and
Trinidad were genetically and morphologically determined to be P.
major subsp. intermedia (syn. P. intermedia DC.).
Although it was not the goal of their study to characterize the global
distribution for each of the subspecies or retrace origins of introduced
populations, their sampling demonstrates that P. major subsp.intermedia is found in some regions of the introduced range, such
as the more southerly latitudes in North America and the Caribbean. More
intensive population sampling around the Middle East and California
would be needed to further narrow down the native origins and further
resolve ancestry and distribution for this Mediterranean ecotype ofP. major and other Plantago species.
Prior adaptation is key to global success
Plantago major is a classic example of a widely-introduced
species for which its successful colonisation and spread in new lands is
not compromised by low genetic variation or the lack of sexual
recombination (Beaumont et al., 2009; Dormontt et al, 2014). Based on
the distribution of ecotypes that have spread to the introduced parts of
the range, and the division seen along the 35–40o N
latitude range, we propose that prior adaptation to climate in multiple
successful ecotypes may explain the distribution and persistence of this
global weed.
When introduced species successfully colonize new ranges, they either do
so by quickly adapting to new conditions, or they arrive with prior
adaptations that arose in their ancestral native range before
introduction (Rey et al., 2012; Jackson et al., 2015; Barrett, 2015;
Bock et al., 2015). The pattern we see in the global ecotype
distribution where the majority of the global populations we sampled
suggests that the successful colonisation of new ranges by P.
major is dependent on plants having a prior adaptation to climate
before arriving to new regions worldwide. Such prior adaptation to
climate has been noted for other species with successful colonists, i.e.
in plants (Sexton et al., 2002; Dlugosch and Parker, 2007; Schlaepfer et
al., 2010; Rosche et al., 2018), birds (Jackson et al., 2015), and
beetles (Vahsen et al, 2018). Although climatic conditions are quite
different between regions occupied by populations belonging to the same
ecotype (i.e., semi-tropical Florida and southern Spain), it is possible
that further climatic niche shifts occurred after introduction, and
although such shifts are thought to be rare in terrestrial plants
(Petitpierre, 2012), it has been demonstrated for other species (i.e.
Jackson et al., 2015). Alternatively, prior adapted lineages could have
evolved tolerance to a large climatic amplitude, and therefore climatic
and environmental variation deserves further investigation.
Although very little is known about the ecological habitats of P.
major in pre-colonial times, the ecotypes in its native range may have
evolved anthropogenically-induced adaptations to invade (AIAI;
Theoharides et al., 2007; Rey et al. 2012; Hufbauer et al., 2012).
Plants like P. major that occupy a broad range of habitat
conditions in their native range, including those that are human-made
and disturbed habitats, can naturalize more easily in new regions
(Hufbauer et al., 2012). Human activity and disturbance have created
conditions that are now more similar across broad geographic regions,
and this homogenization of the environment likely facilitated the
colonisation and successful spread of disturbance tolerant species likeP. major (van Dijk & van Delden, 1981; Estoup & Guillemaud,
2010; Kalusova et al, 2017).
Asexual plants have been found to change just as often and as fast as do
sexual plants when introduced to a new range (Dalrymple et al, 2015).
The extraordinary phenotypic plasticity known in P. major is
likely important for ecotypes coping with a high level of environmental
and climate heterogeneity (Warwick & Briggs, 1980; Samuelsen, 2000).Plantago major can vary extensively in morphology even within
populations, and therefore high phenotypic variation for the species is
not related to genetic diversity at least within a population level,
though further studies are needed (Mølgaard, 1976; Lotz and Blom, 1986;
Wolff, 1991; Iwanycki Ahlstrand et al., 2018). Wide environmental
tolerance and an ability to grow in a multitude of climatic and edaphic
conditions as a result of well-developed phenotypic plasticity, makesPlantago major a classic example of a weed that possesses the
“general purpose genotype strategy” as coined by Baker (1965) – in
this case, it appears that as few as three general-purpose genotypes or
ecotypes may have been involved in the global success of the common
plantain.
Conclusions
Population genomic approaches can provide useful data where historical
records are lacking in helping to retrace migration routes of historical
introductions of plants, such as Plantago major, and improve our
understanding of the role of genomic variation in explaining success of
globally distributed species. Our findings show that beyond
human-mediated migration, factors such as prior adaptation to climate
and anthropogenic disturbance may be key to success for a worldwide
weed. That is, a genotype introduced into a new range with environmental
conditions for which it is already adapted is more likely to succeed.
This suggests that even if globalization creates more opportunities for
species invasions, not all species with invasive potential will be as
likely to succeed in a new area. The six ecotypes we identify serve as
an excellent starting point for future ecological and genomic studies,
and the distribution of genomic diversity across the globe provides a
glimpse into the complex interactions between the environment and the
genome that influence the distribution of plant species and mediate
phenotypic adaptation to local conditions (Bragg et al. 2015).
Introduced plants that exhibit high phenotypic plasticity are
hypothesized to perform better under changing climatic conditions,
despite low genetic variation and sexual recombination. Therefore, our
findings can be used to advance our ecological and evolutionary
understanding of successful invasions, and contribute to more accurately
predicting species responses to global change (Chapman et al., 2016;
Klonner et al., 2017).