PLoS One. 2015; 10(11): e0143478.
Published online 2015 Nov 23. doi: 10.1371/journal.pone.0143478
PMCID: PMC4657909
Tzen-Yuh Chiang, Editor
This article has been cited by other articles in PMC.
Abstract
The intensively discussed taxonomic complexity of the Dactylorhiza
genus is probably correlated with its migration history during
glaciations and interglacial periods. Previous studies on past processes
affecting the current distribution of Dactylorhiza species as
well as the history of the polyploid complex formation were based only
on molecular data. In the present study the ecological niche modeling
(ENM) technique was applied in order to describe the distribution of
potential refugia for the selected Dactylorhiza representatives
during the Last Glacial Maximum. Additionally, future changes in their
potential habitat coverage were measured with regard to three various
climatic change scenarios. The maximum entropy method was used to create
models of suitable niche distribution. A database of Dactylorhiza
localities was prepared on the grounds of information collected from
literature and data gathered during field works. Our research indicated
that the habitats of majority of the studied taxa will decrease by 2080,
except for D. incarnata var. incarnata, for
which suitable habitats will increase almost two-fold in the global
scale. Moreover, the potential habitats of some taxa are located outside
their currently known geographical ranges, e.g. the Aleutian Islands,
the western slopes of the Rocky Mountains, Newfoundland, southern
Greenland and Iceland. ENM analysis did not confirm that the Balkans,
central Europe or central Russia served as the most important refugia
for individual representatives of the Dactylorhiza incarnata/maculata
complex. Our study rather indicated that the Black Sea coast, southern
Apennines and Corsica were the main areas characterized by habitats
suitable for most of the taxa.
Introduction
The Last Glacial Maximum (LGM) refers to the period between 26,500 and 20,000 years ago [1]
that greatly affected the distributions and population sizes of many
temperate plant species. Migration routes and the history of
colonization after the LGM have been studied for various taxa, e.g. Viola rupestris [2], Lathyrus vernus [3], Silene dioica [4], Calluna vulgaris [5], and Betula pendula [6].
For a long time, it has been commonly assumed that during the LGM a lot
of temperate plant species survived within refuge areas in the Balkan,
Apennine and Iberian Peninsulas and in the Caspian and Caucasian regions
(“the southern refugia hypothesis” [7–8].
It has also been established that the general view of high genetic
diversity and haplotype richness in refugial areas in the south is the
result of refugial persistence and accumulation of genetic variation
during ice ages, in comparison with low diversity in glaciated areas in
the north. Populations in previously glaciated areas are genetically
depleted as a consequence of rapid postglacial colonization and the
repeated bottleneck effect during stepwise migration [9–10].
The hypothesis is just a general concept and the individualistic nature
of species' responses to climate change implies that the location of
refugia varies according to the climatic conditions preferences as well
as to the way individual species or populations adapt [11].
The incoming evidence suggests that the southern refugia for the
temperate species were complemented by more northern refugia during the
LGM. "The northern refugia hypothesis" assumes more complex patterns for
the distribution of genetic diversity, where suitable niches were also
distributed much more widely in Europe during the LGM, not only across
Southern Europe, but also in Central Europe close to the line of the ice
sheet [12–13]. This proposal has also been indicated in phylogeographic studies of selected Dactylorhiza species [14].
Dactylorhiza Neck. ex Nevski is a temperate orchid genus which includes taxa of various ploidy levels [15–18]. They are either diploids (2n = 40) or tetraploids (2n = 80). Most currently recognized Dactylorhiza species belong to the Dactylorhiza incarnata/maculata polyploid complex. The most problematic taxa within this complex belong to D. majalis s.l., which evolved by multiple and independent hybridization events between two broadly defined parental lineages: D. incarnata s.l.—recognized as the paternal lineage and D. maculata s.l.—considered to be the maternal lineage [18–29].
The taxonomic complexity of this genus is probably due to its migration
history during glaciations and interglacial periods, as well as
polyploidization episodes, which took place several times e.g. [18,24,28–30].
As assumed by Hedrén et al. [31],
this complex must have originated before the Weichselian glaciation and
its representatives are now distributed across Europe and Asia Minor [32–33]. Within this range, the allotetraploids often occupy limited occupancy areas [17,34]
and many of them are restricted to those regions in more northern or
western Europe that were completely covered by the ice sheet during the
Weichselian glaciation. It has been postulated that numerous
allotetraploid species evolved after the ice age on several, independent
occasions by repeated local polyploidization events in areas where they
are currently found (e.g. [24,31]). This hypothesis has been supported by molecular data, including allozyme variation [18–19,20,35] and AFLPs [25]. However, analysis of plastid DNA [26,28,36]
has disclosed that some variants within the allotetraploids have not
been encountered in the extant parental lineages, indicating that the
allotetraploid complex may also include older taxa which currently
remain unknown.
The Dactylorhiza incarnata/maculata
polyploid complex constitutes an extremely dynamic model of polyploid
speciation and extinction, in which polyploid species evolve
continuously from the same set of broadly defined parental lineages. The
pattern of colonization inferred for the complex representatives seems
to be unusual compared with most other temperate taxa, where polyploids
have proven to be strong colonizers of Arctic regions [37], whereas their diploid progenitors have remained much further south [28].
As is the case with other orchids, Dactylorhiza
is not present in fossil material. Previous studies on past processes
affecting the present distribution of the genus representatives as well
as the history of the formation of polyploid complex have been based
only on molecular data.
Environmental niche models,
which are generated by combining species occurrence records (and/or
absence data) with environmental GIS data layers have become
increasingly important tools to address various issues in biogeography,
ecology, evolution and conservation biology research [38–39]. Many methods have been used for modeling of species distribution (eg. BIOCLIM [40–41], DOMAIN [42], GLM [43], MaxEnt [44]). MaxEnt is considered as the most reliable machine learning programme in computing presence-only data (e.g. [45–48]).
This application is particularly useful in the course of determining
locations of glacial refugia of plants and animals, especially when the
fossil material is poor (e.g. [49–51]. Nowadays, when the rate of climate change accelerates, it is increasingly important to understand the consequences [52]
and here species distribution models based on current ecological niche
constraints are used to project future species distributions (e.g. [53–55]).
While
the frequent niche shifts in the polyploid complexes could be expected,
the evidences for alternative patterns were reported. Findings
published in the recent studies suggest that the niche conservatism may
be more common between different cytotypes than previously recognized.
The tendency for the niche of a taxon to be little changed over time was
confirmed for several polyploid species complexes, i.e. Claytonia perfoliata (Portulacaceae) [56], Larrea tridentata (Zygophyllaceae) [57], Houstonia (Rubiaceae) [58] and Heuchera cylindrica (Saxifragaceae) [59].
Based on the frequent spatial segregation of the diploid and the
polyploid cytotypes, and considering the fact that the polyploidization
may drive the ecological divergence, species distribution modeling seems
to be adequate approach for studies on biogeography of polyploid
complexes.
Because no study revealed any ecological shifts within Dactylorhiza
species so far, we assume that their niches have remained unchanged
since the LGM and they will not transform in the predictable future. The
quantified niche can be therefore projected across a geographic area
for the purposes of mapping applicable climatic conditions for studied
taxa and predicting its potential distribution [60].
In our research, the ecological niche modeling (ENM) technique has been
applied in order to estimate distribution of suitable niches for three
interesting Dactylorhiza species groups (D. incarnata, D. maculata and D. majalis)
during the LGM and in the predictable future. Noteworthy, the studied
tetraploid taxa represent fixed hybrids between known parental lineages.
They should be therefore considered as separated entities with their
own evolutionary history and characterized by specific habitat
requirements. The aim of the study was to confront the phylogeographic
insights into distribution of glacial refugia with the outcomes of the
climate envelope models as well as to evaluate the future changes in the
potential habitat coverage of the studied orchids.
