Reproductive barriers and hybridity in two spruces, Picea rubens and Picea mariana, sympatric in eastern North America

Abstract

Hybridization between red spruce (Picea rubens Sarg.) and black spruce (Picea mariana (Mill.) BSP), lateand early-successional species, respectively, has resulted in identification and management problems. We investigated the nature and magnitude of reproductive and life-cycle success barriers in controlled intra-and inter-specific crosses of red and black spruce. We quantified a number of reproductive, germination, phenological, and performance traits, and examined traits by parental pedigree and hybrid index. Species' pollen had no effect on number of aborted or nonpollinated ovules. Controlled intraspecific crosses had, on average, 6.6 times more filled seeds than interspecific crosses. Cone and seed morphometric traits were species specific, with seed traits showing negative hybridization effects on both species. Germination, cotyledon number, and seedling height had significant species-specific traits, with hybrids showing an additive or slightly negative heterosis. Severe, negative heterosis appears to be of limited importance as an isolating barrier between red and black spruce. Reproductive phenology was remarkably similar among species and hybrid progenies when grown in common garden experiments. Crossability barriers are clearly paramount in maintaining the separation of the species. Ecological separation based on ecophysiological differences (e.g., shade tolerance) also represents an important prezygotic barrier for minimizing the negative effects of hybridization (e.g., postzygotic inviability) on reproductive fitness.Key words: cone, seed and seedling traits, genetic variation, phenology, red and black spruce. Les pollens des espèces sont sans effet sur le nombre d'ovules non-fécondées ou avortées. En conditions contrôlées, les croisements intraspécifiques donnent en moyenne 6,6 fois plus de graines pleines que les croisements interspécifiques. Les caractères morphométriques des cônes et des graines sont spécifiques à l'espèce, les caractères des graines montrant des effets négatifs d'hybridation chez les deux espèces. La germination, le nombre de cotyles et la hauteur des plantules montrent des caractères significativement spécifiques à l'espèce, les hybrides montrant une hétéro-sis additive ou légèrement négative. L'hétérosis négative sévère semble être d'importance limitée comme facteur de barrière d'isolation entre les épinettes rouges et noires. La phénologie de la reproduction est remarquablement similaire entre les espèces et les progénitures hybrides, lorsqu'on les cultive dans des jardins expérimentaux communs. Les barrières aux croisements jouent clairement un rôle primordial dans le maintien de la séparation des espèces. La sépara-tion écologique basée sur des différences écophysiologiques (p. ex., tolérance à l'ombre) représente également une importante barrière pré-zygotique dans la limitation des effets négatifs de l'hybridation (p. ex., mortalité postzygotique) sur l'adaptation reproductive.

New insights on the speciation history and nucleotide diversity of three boreal spruce species and a Tertiary relict

“…Second, IM models further assume a two-species system, where there is no gene flow with any other species. This is clearly not the case in this study, where at least the four included species seem to have exchanged genes with each other and with species not considered here: gene flow between P. glauca and P. engelmanii, and between P. mariana and P. rubens has been documented (for example, Perron and Bousquet, 1997;Major et al, 2005), and P. abies and P. obovata have a large hybrid zone centred around the Ob river (Tollefsrud et al, 2008). However, because P. engelmanii can be considered as a subspecies of P. glauca (Rajora and Dancik, 2000) and P. rubens a derivative of P. mariana (Perron et al, 2000), putative gene flow between them and the species studied here might have a minor effect on the level of polymorphism of the latter.…”

Genetic variation and control of chloroplast pigment concentrations and related needle-level traits in Picea rubens, Picea mariana, and their hybrids: moisture and light environmental effects

“…It should be noted that the hybrid complex would eventually be displaced by the late-successional species under a disturbance regime dominated by gap-replacement dynamics (rather than the catastrophic disturbance associated with fire in boreal environments) capitalizing on the strength of all its adaptive traits (e.g., shade tolerance, lower shoot:root ratio). Crossability and ecological barriers appear most important in maintaining the separation of sympatric RS and BS (Major et al 2005). Interspecific crossability (0-4%) was about seven times lower than among intraspecific crosses, and represents a substantial but imperfect reproductive barrier for maintaining the separation of the species (Major et al 2005).…”

“…The early and late-successional hybridization theory advanced by Petit et al (1997) suggests that this type of hybridization can be a step in the recolonization of late-successional species in their northward migration. An identical reproductive phenology, differential crossability (Major et al 2005), and predominant paternal inheritance of shade-tolerant adaptive traits such as light processing and chlorophyll concentration make this late-successional migration theory plausible for these spruces. It should be noted that the hybrid complex would eventually be displaced by the late-successional species under a disturbance regime dominated by gap-replacement dynamics (rather than the catastrophic disturbance associated with fire in boreal environments) capitalizing on the strength of all its adaptive traits (e.g., shade tolerance, lower shoot:root ratio).…”

Genetic Variation in Picea mariana × P. rubens Hybrid Populations Assessed with ISSR and RAPD Markers

“…Estimates of the degree of natural hybridization or introgression between the two species vary from extensive to minor [4]. Controlled crosses by Manley [5] and Gordon [6] indicated reproductive barriers to hybridization; however, no record of the reproductive phenological barrier between these closely related species is reported in the literature.…”