Following on from the extensive ecological work of Professor Donald Pigott, and using highly polymorphic microsatellite markers, the focus of my research is the molecular ecology and evolutionary genetics of the non-model yet significantly important genus, Tilia (lime trees).
Tilia sp. are widely distributed throughout UK and Europe. Presently two species are predominant, T. cordata Mill. (Small-leaved lime), and T. platyphyllos Scop. (Large-leaved lime). Tilia x europaea L. (Common lime), is a naturally occurring hybrid of the two species. In the UK, Tilia populations are highly fragmented. To advise on the potential management of the two species, we investigated their genetic differentiation and population structure. While morphology can sometimes distinguish UK Tilia, the main features are not always obvious. Microsatellite markers have successfully discriminated between the species and the hybrids. We observed significant differentiation indicating two distinct biological units. Sub-structure within species is loosely related to geographic location and any future conservation efforts should consider provenance issues (Logan et al., 2015).
Tilia sibirica Bayer (Siberian lime), is endemic to the low mountain system north of the Altai Mountains of southern Siberia, approximately 1,000kms to the east of the natural range limit of T. cordata. To date no genetic analyses have been carried out, leaving questions unanswered regarding its genetic status. We used standard population genetic and Approximate Bayesian Computation (ABC) analyses to determine the genetic and demographic history of the Siberian lime and that of its closest congeneric, the small-leaved lime (T. cordata) from Siberia. We showed that the two are distinct biological units with significant genetic differentiation and, based on DIYABC analyses of the microsatellite markers, suggest their Middle Pleistocene divergence. We have shown that the Siberian lime has low within-population genetic diversity as well as high levels of clonality (Logan et al., 2018).
With current climatic induced changes in species distribution, the range-edge populations are important sources of genetic material for either natural or assisted migration. Two aspects are of particular importance, namely, clonal reproduction and mating between related individuals. We have assessed the genetic diversity, relatedness and reproductive strategy of two temperate forest tree species, Tilia cordata and T. platyphyllos, from central and range-edge locations, across much of their European range. It is thought that these species regularly propagate asexually. In this study we discuss the level of clonal reproduction, range effects, relatedness, genetic diversity and effective population size of the two species across much of their range (Logan et al., 2019).
ON-GOING WORK:
We are continuing with our work on the European phylogeography and a more informative phylogenetic study of the genus.
We are further investigating the taxonomic status of the Siberian Lime. This work will be in collaboration with Dr Milan Chytrý of Masaryk University, Czech Republic. While we have shown that T. sibirica is significantly genetically different to T. cordata, we will be using NGS techniques to determine how closely or different the two taxa are.
We are also continuing to work on our RNA-seq data to construct a de novo assembly of T. cordata and T. platyphyllos transcriptomes.
As well as being a naturally occurring organism, Tilia x europaea has been planted in parks and gardens since the 17th Century, especially as ornamental trees lining ancient avenues. Identification of which will contribute to the restoration and conservation of ancient lime avenues, ensuring these historical sites remain a part of our heritage. Cultivars from popular clones of Common lime are still widely sourced and planted throughout the UK and Europe. As a result, we are constructing a genetic database of the most commonly available cultivar types. we have collaborated with the National Trust, the Royal Parks and other landowners to genetically catalogue the Tilia species and cultivar types that are present on their land. This work is a collaboration with Dr Sarah Couch and other historic landscape architects to identify ancient Common lime (Tilia x europaea) cultivars across Europe (Wolff et al., in prep).