A last stand in the Po valley: genetic structure and gene flow patterns in Ulmus minor and U. pumila

Abstract

Background and aims: Ulmus minor has been severely affected by Dutch elm disease (DED). The introduction into Europe of the exotic Ulmus pumila, highly tolerant to DED, has resulted in it widely replacing native U. minor populations. Morphological and genetic evidence of hybridization has been reported, and thus there is a need for assessment of interspecific gene flow patterns in natural populations. This work therefore aimed at studying pollen gene flow in a remnant U. minor stand surrounded by trees of both species scattered across an agricultural landscape.

Methods: All trees from a small natural stand (350 in number) and the surrounding agricultural area within a 5-km radius (89) were genotyped at six microsatellite loci. Trees were morphologically characterized as U. minor, U. pumila or intermediate phenotypes, and morphological identification was compared with Bayesian clustering of genotypes. For paternity analysis, seeds were collected in two consecutive years from 20 and 28 mother trees. Maximum likelihood paternity assignment was used to elucidate intra- and interspecific gene flow patterns.

Key results: Genetic structure analyses indicated the presence of two genetic clusters only partially matching the morphological identification. The paternity analysis results were consistent between the two consecutive years of sampling and showed high pollen immigration rates (∼0·80) and mean pollination distances (∼3 km), and a skewed distribution of reproductive success. Few intercluster pollinations and putative hybrid individuals were found.

Conclusions: Pollen gene flow is not impeded in the fragmented agricultural landscape investigated. High pollen immigration and extensive pollen dispersal distances are probably counteracting the potential loss of genetic variation caused by isolation. Some evidence was also found that U. minor and U. pumila can hybridize when in sympatry. Although hybridization might have beneficial effects on both species, remnant U. minor populations represent a valuable source of genetic diversity that needs to be preserved.

Keywords: Ulmus minor; Ulmus pumila; biological invasion; conservation genetics; elm; forest remnant; fragmentation; genetic diversity; habitat degradation; hybridization; long distance dispersal; paternity analysis; plain forest..

Fig. 1.

Sampling area surrounding the relic Porporana stand. Different symbols distinguish mother trees (i.e. trees from which seeds were harvested) from all other adults trees sampled (see key). Different shadings distinguish U. minor (white) from U. pumila (black) and putative hybrid individuals (grey) according to morphological criteria (see Materials and methods).

Fig. 2.

Results of cluster analyses. (A) Principal coordinates analysis scatterplot, with different shadings indicating different clusters according to STRUCTURE analysis and different symbols indicating morphological identification. (B) The same shading and symbol representation plotted on the geographical position of individuals, zooming in on the central Porporana stand (C). (D) Distribution of q values from STRUCTURE analysis for the Porporana stand (on the left) and the surrounding agricultural area (on the right). The black and white portions of each bar are the probabilities of belonging to cluster A and B, respectively. Grey dotted lines represent 0·8 thresholds applied for assignment to clusters A and B.

Fig. 3.

Number of pollen donors from different clusters (according to STRUCTURE analysis) in relation to their reproductive success in terms of the number of sired seeds in 2007 and 2008.

Fig. 4.

Distribution of observed pollen dispersal distances (black bars) and distribution of all distances between potential pollen donors and maternal trees (open bars) in 2007 and 2008.