Selection on hybrids of ecologically divergent ecotypes of a marine snail: the relative importance of exogenous and endogenous barriers

Abstract

Unravelling the form of selection acting on hybrids of ecotypes undergoing ecological speciation is essential to understand the mechanisms behind the evolution of reproductive isolation in the face of gene flow. Shell phenotype is known to be affected by natural selection and is involved in the fitness of the marine snail Littorina saxatilis. Here, we studied the association between shell traits and fitness in hybrids in order to determine the relative role of exogenous and endogenous selection in this hybrid zone of L. saxatilis. We show that directional selection is the predominant mode of selection among hybrids. We also show its heterogeneity, affecting different shell traits, within populations at the level of the microhabitat. Therefore, endogenous selection mechanisms are most probably lacking in this hybrid zone and exogenous barriers (pre‐ and post‐zygotic) are possibly one of the main forces behind the evolution of barriers to gene flow between these ecologically divergent ecotypes. This study shows how this barrier might represent an important type of reproductive isolation within ecological speciation, and this should be taken into account in future studies of speciation in hybrid zones. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 391–400.

Ecological divergence plays an important role in strong but complex reproductive isolation in campions (Silene)*

“…Primarily intrinsic barriers, in contrast, such as conspecific pollen precedence (Rahmé et al 2009) and reductions in F 1 pollen viability (Brothers and Delph 2010), had moderate individual strengths but contributed only little to total reproductive isolation. Similar patterns were detected in a number of other plant and animal systems and taken as evidence for ecological speciation (e.g., Galindo et al 2014;Baack et al 2015;Sobel and Streisfeld 2015;Ostevik et al 2016;Richards et al 2016;Lackey and Boughman 2017;Cahenzli et al 2018;Paudel et al 2018). Below we discuss probable drivers of speciation in Silene, relate our findings to estimates of migration rate (Muir et al 2012;Hu and Filatov 2015) and explore the role of variation in RI for gene flow.…”

“…Similar patterns were detected in a number of other plant and animal systems and taken as evidence for ecological speciation (e.g., Galindo et al. ; Baack et al. ; Sobel and Streisfeld ; Ostevik et al.…”

“…This scenario is supported in many systems with weak intrinsic postzygotic barriers but strong prezygotic and extrinsic postzygotic barriers (Nosil 2012;Seehausen et al 2014). In animals, the habitat choice and/or sexual isolation are reported as drivers of speciation, for example in Heliconius butterflies (Mavarez et al 2006), Littorina snails (Galindo et al 2014), and sticklebacks (Lackey and Boughman 2017). In plants, for example in taxon pairs of the genera Mimulus, Helianthus, Senecio, Petunia, Dianthus, and Roscoea, strong reproductive isolation resulted from adaptive divergence and/or pollinator isolation and differences in flowering time (Lowry et al 2008;Dell'Olivo et al 2011;Baack et al 2015;Sobel and Streisfeld 2015;Ostevik et al 2016;Richards et al 2016;Cahenzli et al 2018;Paudel et al 2018).…”

“…), Littorina snails (Galindo et al. ), and sticklebacks (Lackey and Boughman ). In plants, for example in taxon pairs of the genera Mimulus , Helianthus, Senecio, Petunia, Dianthus , and Roscoea , strong reproductive isolation resulted from adaptive divergence and/or pollinator isolation and differences in flowering time (Lowry et al.…”

Diploid hybrid origin ofHippophaë gyantsensis(Elaeagnaceae) in the western Qinghai-Tibet Plateau

“…One potential outcome of hybridization is the creation of novel polyploid or homoploid species through allopolyploid speciation or homoploid hybrid speciation, respectively (Abbott et al, 2013;Kolar cik et al, 2014). Unlike allopolyploid speciation, in which genome duplication can create a strong postzygotic reproductive barrier between a hybrid and its parents, homoploid hybrid speciation does not involve ploidy changes in the hybrid (Mallet, 2007;Abbott et al, 2010Abbott et al, , 2013Galindo et al, 2014). In the absence of intrinsic isolation caused by chromosomal and/or genetic sterility barriers, ecological and spatial isolation are therefore critical in allowing homoploid hybrid species to escape the homogenizing effects of gene flow from parental species and eventually to become fully stabilized both genomically and phenotypically (Rieseberg, 1997;Buerkle et al, 2000;Gross & Rieseberg, 2005;Abbott et al, 2010).…”