Thank you for not flowering: conservation genetics and gene flow analysis of native and non-native populations of Fraxinus (Oleaceae) in Ireland

Abstract

The risks of gene flow between interfertile native and introduced plant populations are greatest when there is no spatial isolation of pollen clouds and phenological patterns overlap completely. Moreover, invasion probabilities are further increased if introduced populations are capable of producing seeds by selfing. Here we investigated the mating system and patterns of pollen-mediated gene flow among populations of native ash (Fraxinus excelsior) and mixed plantations of non-native ash (F. angustifolia and F. excelsior) as well as hybrid ash (F. excelsior  F. angustifolia) in Ireland. We analysed the flowering phenology of the mother trees and genotyped with six microsatellite loci in progeny arrays from 132 native and plantation trees (1493 seeds) and 444 potential parents. Paternity analyses suggested that plantation and native trees were pollinated by both native and introduced trees. No signs of significant selfing in the introduced trees were observed and no evidence of higher male reproductive success was found for introduced trees compared with native ones either. A small but significant genetic structure was found (^f t ¼ 0.05) and did not correspond to an isolation-by-distance pattern. However, we observed a significant temporal genetic structure related to the different phenological groups, especially with early and late flowering native trees; each phenological group was pollinated with distinctive pollen sources. Implications of these results are discussed in relation to the conservation and invasiveness of ash and the spread of resistance genes against pathogens such as the fungus Chalara fraxinea that is destroying common ash forests in Europe.

Biological Flora of the British Isles:Fraxinus excelsior

“…The introduced trees did not have a reproductive advantage over native trees so hybrid genes will not be preferentially spread (Thomasset et al . (). This hybrid is considered undesirable for forestry since F. angustifolia has poorer timber characteristics (Fernández‐Manjarrés et al .…”

“…The greater distance than recorded by Heuertz et al (2003) was attributed to the more open nature of the fragmented wooded landscape facilitating airborne pollen movement and alleviating the otherwise detrimental genetic effects of population fragmentation. In the open Irish landscape, Thomasset et al (2014) found a mean pollen spread of 260-382 m with a maximum (detected from tree parentage) of >4000 m. However, the relative contribution of pollen and seeds to gene flow within and between populations depends upon whether it is a mast year (Bacles, Lowe & Ennos 2006;Bacles & Ennos 2008); in a mast year, seed dispersal is up to six times more effective in moving genes than pollen dispersal between fragmented populations.…”

“…In the open Irish landscape, Thomasset et al . () found a mean pollen spread of 260–382 m with a maximum (detected from tree parentage) of >4000 m. However, the relative contribution of pollen and seeds to gene flow within and between populations depends upon whether it is a mast year (Bacles, Lowe & Ennos ; Bacles & Ennos ); in a mast year, seed dispersal is up to six times more effective in moving genes than pollen dispersal between fragmented populations.…”

“…Most of these hybrids were cryptic and could help explain the very variable performance of plantation ash in Ireland. The introduced trees did not have a reproductive advantage over native trees so hybrid genes will not be preferentially spread (Thomasset et al (2014). This hybrid is considered undesirable for forestry since F. angustifolia has poorer timber characteristics (Fern andez-Manjarr es et al 2006).…”

Genetic analysis of European common ash (Fraxinus excelsior L.) populations affected by ash dieback

“…HÖLTKEN et al, 2003;MORAND et al, 2002;HEUERTZ et al, 2001HEUERTZ et al, , 2003HEUERTZ et al, , 2004aHEUERTZ et al, , 2004bHEUERTZ et al, , 2006HEBEL et al, 2006aHEBEL et al, , 2006bFERRAZZINI et al, 2007;BACLES and ENNOS, 2008;SUTHERLAND et al, 2010;GÖMÖRY et al, 2012), others studies are dealing with gene flow and hybridization (e.g. FERNÁNDEZ-MANJARRÉS et al, 2006;THOMASSET et al, 2013THOMASSET et al, , 2014GÉRARD et al, 2013). In Southern Germany one study on genetic variation of European ash focus on provenance regions (HEBEL et al, 2006b).…”