Costs and benefits of the mixed-mating system of Narcissus serotinus (Amaryllidaceae) in the conservation of small fragmented populations

Small fragmented populations often exhibit reduced plant-pollinator interactions and scarce outcrossing opportunities. In this context, mixed-mating systems can be advantageous since selfing can provide reproductive assurance, but they may also carry relevant costs such as those involved in inbreeding depression. This study examines the advantages and costs of selfing in Narcissus serotinus L., a mixed-mating geophyte that currently occurs in several fragmented populations in Portugal, that resulted from the construction of the largest European dam. Observation of pollinators revealed that cross-pollination is less frequent in small than in large populations. Manual self-pollinations significantly increased fruit and seed set in small-size populations, also suggesting limited insect pollination. The existence of selfing may provide reproductive assurance in small-size populations of N. serotinus where outcrossing pollination is reduced. Although floral biology and experimental pollinations showed that N. serotinus is capable of autonomous selfing, four of the six fitness traits studied showed significant inbreeding depression in all populations. The high levels of inbreeding depression found in N. serotinus suggest that the initial reproductive assurance advantage of selfing may be counterbalanced by lower survival of the resulting individuals and a decrease in the evolutionary potential of the populations. Key words: fragmentation, human disturbance, inbreeding depression, mixed-mating system, selfing. Les petites populations fragmentees presentent souvent des interactions plante-pollinisateur reduites et des opportunites de croisement eloigne rares. Dans ce contexte, les systemes de croisement mixtes peuvent etre avantageux car l'endogamie peut fournir une assurance reproductive, mais elle peut aussi comporter des couts relies tels ceux impliques dans la depression de consanguinite. Cette etude a examine les avantages et les couts de l'endogamie chez Narcissus serotinus L., un geophyte a systeme mixte de reproduction qui apparait actuellement chez plusieurs populations fragmentees au Portugal, a la suite de la construction du plus grand barrage en Europe. L'observation des pollinisateurs a revele que la pollinisation croisee est moins frequente chez les petites populations comparativement aux plus grandes. L'autopollinisation manuelle augmentait significativement la nouaison et lagrenaison dans les populations de petite taille, suggerant aussi une pollinisation par les insectes reduite. L'existence d'endogamie peut fournir une assurance reproductive dans les populations de petite taille de N. serotinus ou la pollinisation par croisement eloigne est reduite. Meme si la biologie florale et les pollinisations experimentales ont montre que N. serotinus est capable d'endogamie, quatre de six traits de fitness etudies ont montre une depression de consanguinite significative chez toutes les populations. Les hauts niveaux de depression de consanguinite trouves chez N. serotinus suggerent que l'avantage initial de l'assurance reproductive de l'endogamie peut etre contrebalance par une survie plus faible des individus qui en resultent et une diminution du potentiel evolutionnaire des populations. [Traduit par la Redaction] Mots-cles: fragmentation, perturbation anthropique, depression de consanguinite, systeme de reproduction mixte, endogamie.
Introduction A general effect of human activities is that natural habitats are frequently lost or changed (Allendorf and Luikart 2007). These human-induced changes may affect persistence and survival of species, [...]