Breeding systems and continuing evolution in the endemic Sorbus taxa on Arran

Abstract

The Arran whitebeams Sorbus arranensis and S. pseudofennica are two endemic woody plant taxa that have evolved on Arran through hybridisation. S. arranensis is a triploid hybrid between the widespread diploid S. aucuparia and the rare tetraploid S. rupicola. S. pseudofennica is a tetraploid formed by crossing between S. arranensis and S. aucuparia. In order to determine the mating systems of the two endemic species six maternal trees of each taxon together with 10-12 of their seed offspring were scored for their phenotype at three microsatellite loci and one nuclear intron locus. All seeds of S. arranensis were identical in phenotype to their maternal parents. In S. pseudofennica, 17.5% of all seeds differed in marker phenotype from their maternal parent. The proportion of seed with nonmaternal phenotypes varied significantly among maternal trees of S. pseudofennica. The results suggest that the triploid S. arranensis is an obligate apomict, whereas the tetraploid S. pseudofennica is a facultative apomict. Molecular marker analysis of three trees from Arran with an unusual leaf morphology indicates that they are the product of sexual reproduction by S. pseudofennica, and may originate from hybridisation with S. aucuparia. This research demonstrates that the Sorbus taxa on Arran are participants in an active evolutionary process generating novel biodiversity. Conservation programmes for these taxa should aim to preserve this evolutionary process rather than the individual taxonomic entities that it produces.

A novel mating system analysis for modes of self-oriented mating applied to diploid and polyploid arctic Easter daisies (Townsendia hookeri)

“…Ford and Richards (1985) found that in several Taraxacum agamospecies, up to 62% of offspring had isozyme profiles that differed from their mothers. Based on microsatellite gel patterns, Robertson et al (2004) found that progeny of Sorbus arranensis were all identical to their mothers and denoted Figure 2 Log-likelihoods across a range of outcrossing rate t, for other parameters (s, u, a) jointly estimated. Note the sharp crest of likelihoods at low t for the tetraploid populations, while the diploid population (Tantalus Butte) has a much broader crest at a higher t.…”

Conservation of Britain's biodiversity: the case of Hieracium cyathis (Asteraceae) and its relation to other apomictic taxa

“…Conservation strategies have been outlined for other taxonomically complex groups such as Sorbus (Robertson, Newton & Ennos, 2004) or Euphrasia (Ennos et al, 2005) in the British Isles. For these two genera, hybrids are continuously recreated from their diploid parentals in situ.…”

Parentage of endemic Sorbus L. (Rosaceae) species in the British Isles: evidence from plastid DNA

“…On Arran, S. arranensis has back‐crossed with S. aucuparia on at least five occasions to give Sorbus pseudofennica E. F. Warb. (Robertson et al ., 2004a, b), and the plants are tetraploid and reasonably fertile. Sorbus pseudofennica itself has also back‐crossed with S. aucuparia at least twice to form Sorbus pseudomeinichii A. Robertson (Robertson and Sydes, 2006).…”