4.1 Species boundary delimitation of Androsace cantabricausing phylogenomics
Clarifying the taxonomic status of A. cantabrica has both
scientific and conservation implications, as distinguishing it from
closely related taxa is crucial for understanding its threatened status
and prioritizing conservation measures (Godfray et al., 2004; Ottewell
et al., 2016; Kress et al., 2017). We implemented a phylogenomic
approach to resolve whether A. cantabrica is a valid species and,
therefore, threatened or if it is part of a wider and non-threatened
taxon (A. adfinis subsp. adfinis ). Phylogenetic
reconstructions based on hundreds of nuclear loci and plastid sequences
obtained from Angiosperms353 target capture data consistently resolvedA. cantabrica as a monophyletic clade, confirming its
phylogenetic distinctiveness in alignment with previous morphological
(Kress, 1997) and karyotypic data (Kress, 1984). Our phylogenomic
approach improved our understanding of A. cantabrica and allowed
us to investigate its distribution, genetic diversity and evolutionary
origin and supported its categorization as a valid species in need of
conservation.
The placement of A. cantabrica as sister to the /halleri clade in
the nuclear Angiosperms353 phylogenetic trees, rather than to A.
adfinis, aligns with the morphological groupings of Androsace sect. Aretia as per Smith & Lowe (1997). Interestingly, while
nuclear Angiosperms353 data positioned A. cantabrica as a sister
to the /halleri clade, plastid data and previous studies suggested a
closer phylogenetic relation to A. adfinis . This topological
incongruence may reflect a divergence in evolutionary paths between the
plastid and nuclear genomes, a phenomenon observed across many plant
species (e.g., Galbany-Casals et al., 2014; Viruel et al., 2018 Favre et
al., 2022; Liu et al., 2023).
The evolutionary origin and taxonomic identity of A. cantabrica have been subjects of significant debate. Initially, it was hypothesized
that A. cantabrica was an allopolyploid species resulting from
hybridization between A. laggeri (2n = 38, localized in
the central Pyrenees) and A. halleri (2n = 38, distributed
in the Cantabrian Mountains, Pyrenees, Massif Central, and Vosges;
Kress, 1984). However, this hypothesis was challenged by Dixon et al.
(2008), who used amplified fragment length polymorphism (AFLP) data to
refute a close relationship between A. cantabrica and eitherA. halleri or A. laggeri . Instead, they suggested thatA. cantabrica was an autopolyploid related to the southwestern
Alps’ A. adfinis s.l. , which includes A. adfinis subsp.adfinis , A. adfinis subsp. puberula (Jord. &
Fourr.) Kress, and A. adfinis subsp. brigantiaca . Our data
indicate that A. cantabrica is a polyploid and likely has a
complex evolutionary history. While Dixon et al. (2008) proposed an
autopolyploid origin closely related to A. adfinis , our analysis
suggests a different scenario, albeit without conclusively identifying
the exact polyploidy type. Although paralogous genes were limited in our
analysis, which included A. cantabrica samples, definitive proof
of an autopolyploid origin is absent. For example, A. adfinis subsp. brigantiaca is suspected of being a recent hybrid (Boucher
et al., 2016) with tetraploid features, yet only exhibited six
paralogous genes in HybPiper analyses. Possible scenarios for A.
cantabrica origin include an ancient homoploid hybridization event
involving the ancestors of A. cantabrica and the /halleri clade,
followed by local polyploidization, or an initial allopolyploidization.
In any case, given its phylogenetic, morphological, and karyological
uniqueness, A. cantabrica should be considered a valid species
with an evolutionary trajectory shaped by rapid speciation,
introgression, and possibly hybridization in alpine environments
(Hibbins et al., 2020; Smyčka et al., 2022).