Postglacial dispersal, rather than in situ glacial survival, best explains the disjunct distribution of the Lusitanian plant species Daboecia cantabrica (Ericaceae)

Aim:

The distribution of the Lusitanian flora and fauna, species which are found only in southern and western Ireland and in northern Spain and Portugal but which are absent from intervening countries, represents one of the classic conundrums of biogeography. The aim of the present study was to determine whether the distribution of the Lusitanian plant species Daboecia cantabrica was due to persistence in separate Irish and Iberian refugia, or has resulted from post-glacial recolonization followed by subsequent extinction of intervening populations.

Location:

Northern Spain and Co. Galway, western Ireland.

Methods:

Palaeodistribution modelling using Maxent was employed to identify putative refugial areas for D. cantabrica at the Last Glacial Maximum (LGM). Phylogeographical analysis of samples from 64 locations in Ireland and Spain were carried out using a chloroplast marker (atpB–rbcL), the nuclear ITS region, and an anonymous nuclear single-copy locus.

Results:

The palaeodistribution model indicated areas with a high probability of survival for D. cantabrica at the LGM off the western coast of Galicia in Spain, and in the Bay of Biscay. Spanish populations exhibited substantially higher genetic diversity than Irish populations at all three loci, as well as geographical structuring of haplotypes within Spain consistent with divergence in separate refugia. Spanish populations also exhibited far more endemic haplotypes. Divergence time between Irish and Spanish populations associated with the putative Biscay refugium was estimated as 3.333–32 ka.

Main conclusions:

Our data indicate persistence by D. cantabrica throughout the LGM in two separate southern refugia: one in western Galicia and one in the area off the coast of western France which now lies in the Bay of Biscay. Spain was recolonized from both refugia, whilst Ireland was most likely recolonized from the Biscay refugium. On the balance of evidence across the three marker types and the palaeodistribution modelling, our findings do not support the idea of in situ survival of D. cantabrica in Ireland, contrary to earlier suggestions. The fact that we cannot conclusively rule out the existence of a small, more northerly refugium, however, highlights the need for further analysis of Lusitanian plant species.

Phylogeographical analysis of two cold-tolerant plants with disjunct Lusitanian distributions does not support in situ survival during the last glaciation

Aim: We used a combination of modelling and genetic approaches to investigate whether Pinguicula grandiflora and Saxifraga spathularis, two species that exhibit disjunct Lusitanian distributions, may have persisted through the Last Glacial Maximum (LGM, c. 21 ka) in separate northern and southern refugia.

Location: Northern and eastern Spain and south-western Ireland.

Methods: Palaeodistribution modelling using maxent was used to identify putative refugial areas for both species at the LGM, as well as to estimate their distributions during the Last Interglacial (LIG, c. 120 ka). Phylogeographical analysis of samples from across both species' ranges was carried out using one chloroplast and three nuclear loci for each species.

Results: The palaeodistribution models identified very limited suitable habitat for either species during the LIG, followed by expansion during the LGM. A single, large refugium across northern Spain and southern France was postulated for P. grandiflora. Two suitable regions were identified for S. spathularis: one in northern Spain, corresponding to the eastern part of the species' present-day distribution in Iberia, and the other on the continental shelf off the west coast of Brittany, south of the limit of the British–Irish ice sheet. Phylogeographical analyses indicated extremely reduced levels of genetic diversity in Irish populations of P. grandiflora relative to those in mainland Europe, but comparable levels of diversity between Irish and mainland European populations of S. spathularis, including the occurrence of private hapotypes in both regions.

Main conclusions: Modelling and phylogeographical analyses indicate that P. grandiflora persisted through the LGM in a southern refugium, and achieved its current Irish distribution via northward dispersal after the retreat of the ice sheets. Although the results for S. spathularis are more equivocal, a similar recolonization scenario also seems the most likely explanation for the species' current distribution.

The not-so-Irish spurge: Euphorbia hyberna (Euphorbiaceae) and the Littletonian plant ‘steeplechase’

The disjunct distributions of the Lusitanian flora, which are found only in south-west Ireland and northern Iberia, and are generally absent from intervening regions, have been of great interest to biogeographers. There has been much debate as to whether Irish populations represent relicts that survived the Last Glacial Maximum (LGM; approximately 21 kya), or whether they recolonized from southern refugia subsequent to the retreat of the ice and, if so, whether this occurred directly (i.e. the result of long distance dispersal) or successively (i.e. in the manner of a ‘steeplechase’, with the English Channel and Irish Sea representing successive ‘water-jumps’ that have to be successfully crossed). In the present study, we used a combined palaeodistribution modelling and phylogeographical approach to determine the glacial history of the Irish spurge, Euphorbia hyberna, the sole member of the Lusitanian flora that is also considered to occur naturally in south-western England. Our findings suggest that the species persisted through the LGM in several southern refugia, and that northern populations are the result of successive recolonization of Britain and Ireland during the postglacial Littletonian warm stage, akin to the ‘steeplechase’ hypothesis.