Experimental crosses within European Silene latifolia (caryophyllaceae): intraspecific differentiation, distance effects, and sex ratio

The dioecious weed Silene latifolia Poiret is thought to have spread northwards through Europe from separate southern source populations and shows a pronounced east-west pattern of differentiation in seed morphology. We used crossing experiments to investigate whether patterns of interfertility in S. latifolia are consistent with a scenario of ongoing speciation (reflected by outbreeding depression in crosses between the seed races), a scenario involving local inbreeding (reflected by heterosis in interpopulations crosses), or a combination of both scenarios. The experiments involved three western and three eastern populations, which were crossed reciprocally in all possible inter- and intra-population combinations. Inter-race cross-progenies did not have lower fitness than those from intraracial crosses, and the results are not consistent with a scenario of incipient speciation. A pattern of overall heterosis was found in three variables, indicating the expression of inbreeding depression in progeny from intrapopulation crosses. For two fitness variables, negative relationships between interpopulation distance and heterosis, together with signs of outbreeding depression in the longest-distance crosses, suggest that there may be significant levels of genetic differentiation between geographically distant populations. The sex ratio was female-biased in most progenies, especially in those from the longest-distance crosses. Key words: experimental hybridization, heterosis, inbreeding depression, incipient speciation, outbreeding depression, sex ratio. On croft que la plante adventice dioique Silene latifolia Poiret s'est etendue vers le nord de l'Europe a partir de populations sources meridionales distactes et on observe un patron marque est-ouest dans la differenciation de sa morphologie seminale. Les auteurs ont conduit des experiences de croisement pour determiner si les patrons d'interfertilite du S. latifolia concordent avec un scenario de speciation continue (traduit par une depression dans les croisements entre les races de graines), un scenario impliquant un autocroisement local (indique par l'heterosis dans les croisements entre populations) ou une combinaison des deux scenarios. Les experiences impliquent trois populations occidentales et trois populations orientales qu'on a croisees reciproquement selon toutes les combinaisons entre et dans les populations. Les progenitures provenant de croisements entre races ne montrent pas moms d' adaptation que les croisements dans la race, et les resultats ne concordent pas avec un scenario de speciation naissante. On observe un patron d'heterosis general pour trois variables, ce qui indique l'expression dune depression d'autocroisement dans la progeniture provenant de croisements dans la population. Pour deux des trois variables d' adaptation, on observe des relations negatives selon les distances entre populations et l'heterosis, lesquelles, prises ensemble avec les signes de depression par croisements externes sur de longues distances, suggerent qu'il puisse y avoir des degres significatifs de differenciation genetique entre les populations geographiquement eloignees. Les rapports sexuels montrent un bicis en faveur des femelles chez la plupart des progenitures, surtout chez cenes provenant de croisements sur les distances les plus longues. Mots-cles : hybridation experimentale, heterosis, depression d'autocroisement, speciation naissante, depression decoulant du croisement, proportions des sexes. [Traduit par la Redaction]
Introduction Conspecific populations that are separated from each other, for example by geographic distance or physical barriers, may diverge genetically and phenotypically as a result of local selection and (or) [...]