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Measuring genetic variation

In a context of habitat fragmentation, plant populations decrease their size and increase their degree of isolation. From a genetic point of view, genetic diversity of fragmented plant populations is likely to decrease over time. Demographic and genetic mechanisms explain such a decrease in genetic diversity. Small populations with a low effective reproductive size (i.e. the number of reproductive individuals) exhibit high inbreeding and biparental inbreeding that causes inbreeding depression (i.e. the reduction in fitness of self compared to the outcrossed progeny). Inbreeding depression reduces plant fitness components, such as survivorship, growth and reproduction, enhancing therefore population extinction. Isolation increases the distance between populations, decreasing therefore the probability of gene flow between populations in a given area.

Gene flow is effective in preventing inbreeding depression as increases population outcrossing rates. The implications of genetic diversity loss are manifold. For example, inbreeding increases homozygosity of extant individuals and inbreeding depression can eliminate several genotypes from the population. The loss of genetic diversity reduces the genetic potential of plants to adapt to new environmental conditions. In fact, the process of local adaptation to local population characteristics can be strongly enhanced by a situation of high inbreeding and no gene flow. Hence, if plants perform well only in their origin population, colonization of suitable habitats might not be successful. In any case, either progressive fitness decline or excessive adaptation to local conditions can significantly increase extinction probabilities of plant populations in a fragmented landscape.

Genetic diversity represents the base for morphological trait variation on which natural selection acts. If genetic diversity reduces over time due to the factors mentioned above, the possibility to develop well-adapted phenotypes to changing environmental conditions is also reduced. The use of low variable molecular markers can be used to examine the genetic origin of plant populations at the regional scale. In the case of Central and Northern Europe, the current distribution pattern of plant species depends on the interglacial colonization of plant species from glacial refugia in Southern Europe. For this reason, other studies that addressed this question also found low levels of large-scale genetic variation in Northern latitudes.

Chloroplast DNA (cpDNA hereafter) is commonly used in evolutionary and phylogenetic studies given that cpDNA evolves slowly and most cpDNA polymorphisms are thought to be caused by structural rearrangement or mutation. However, several other studies have used cpDNA to assess the large-scale genetic relationship among and between populations of plant species across a considerable part of their distribution area. Genetic variation of several study species (i.e. Succisa pratensis, Hypochaeris radicata, Tragopogon pratensis, Scabiosa columbaria, Pimpinella saxifraga, Ranunculus bulbosus and Carlina vulgaris) was analyzed using all cpDNA markers available (around 15 cpDNA markers) and the method of polymerase chain reaction - restriction fragment length polymorphism (PCR–RFLP). Results showed that, except Pimpinella saxifraga, the rest of plant species exhibited no variation in cpDNA markers. Hence, results suggest that several plant species in Europe could share a common ancestor. In the case of Pimpinella saxifraga, the pattern of variation found (variation within and between regions) could be interpretated as (1) different origins since the last interglacial colonization, or (2) that this species is prone to variation in cpDNA. There are studies supporting each one of these possibilities. The population-level genetic variation requires more specific genetic markers, such as the microsatellites developed for Hypochaeris radicata.

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