A multi-faceted approach for assessing evolutionary significant conservation units in the endangered Omphalodes littoralis subsp. gallaecica (Boraginaceae)

Genetic diversity is now regarded as a key component of biodiversity and its assessment has become a frequent addition to conservation studies. However, due to practical limitations, most studies assess genetic variation using neutral markers while the variability of evolutionary relevant quantitative traits is typically overlooked. Here, we simultaneously assessed neutral and quantitative variation in an endangered plant to identify the mechanism behind their spatial arrangement and to propose conservation guidelines for maximizing mid- to long-term survival. Omphalodes littoralis subsp. gallaecica is a self-fertilizing therophyte with an extremely narrow and fragmented distribution. Regardless of the marker set (non-coding sequences of cpDNA or Amplified Fragment Length Polymorphism loci), the five extant populations of O. littoralis subsp. gallaecica showed minimal to none neutral genetic diversity and a lack of gene flow between them. Moreover, genetic structure was identical in samples collected on two consecutive years suggesting that the seed bank cannot buffer against genetic loss. High rates of self-fertilization together with a strongly fragmented distribution and recurrent bottlenecks seem the likely mechanisms that may have led to a dramatic loss of genetic variation in a classic scenario drawn by genetic drift. Despite the extremely narrow distribution range, reciprocal transplant experiments revealed that the populations differed in several quantitative traits and that these differences likely have a genetic basis. Nevertheless, the pattern of differences among populations did not fit the expectations of local adaptation. Instead, phenotypic variation seemed another outcome of genetic drift with important implications for conservation because each population should be designated as an independent evolutionary significant unit (ESU). Our study evidences the benefits of combining neutral markers with appropriate assessments of phenotype variation, and shows that even endemics with extremely narrow ranges can contain multiple conservation units.