Assessment of natural interspecific hybridization of blue and Engelmann spruce in southwestern Colorado

Abstract

In a partial diallel mating design among 20 blue and 20 Engelmann spruce parents, the interspecific crosses were successful only with Engelmann spruce as the female parent. No viable seed were obtained from the reciprocal cross among the 60 full-sib families attempted. Under the conditions of artificial pollination and a controlled germination environment, an average of 0.3% of the seed germinated on a total seed basis across all 20 Engelmann spruce females. Many abnormalities were observed among the germinating hybrid seed, suggesting hybrid inviability also contributes to the low crossability between these two species. Isozyme analysis was used to confirm the interspecific hybrids between blue and Engelmann spruce based on the unique genotypic compositions of the hybrids relative to the two species. No natural F1 hybrids between blue and Engelmann spruce were observed in this study based on isozyme analysis of mature individuals or their seedling progeny. Analyses included samples of open-pollinated seed from blue and Engelmann spruce females located in an area where both species are present in close proximity, often side-by-side, and where pollen shed and female strobilus receptivity in the two species are coincident. In addition, there was evidence of possible gametic selection or hybrid inviability among the full-sib progeny based on deviations of observed from expected segregation ratios for progeny isozyme genotypes. Deviations tended to favor the allele more common to both species rather than the allele unique or more common to only one species. Also, the elevationally allopatric blue and Engelmann spruce subpopulations were less divergent genetically than the sympatric subpopulations.

Relationships among the Spruces (Picea, Pinaceae) of Southwestern North America

“…Likewise, a common garden test and monoterpene composition unequivocally established this seedlot as P. pungens (Rehfeldt 1994). If hybridization between P. engelmannii and P. pungens occurs in sympatric populations, it has not been detected with molecular markers or common garden studies (Ernst et al 1990;Rehfeldt 1994) and must be an infrequent event.…”

“…Large differences in allele frequencies between P. engelmannii and P. pungens were found by Ernst et al (1990); the species were xed for alternate alleles at some isozyme loci. Four of the loci used by Ernst et al (1990), their Gdh, Got3, Pgi2, and Pgm, are common to our study.…”

“…For example, because of the dif culty in identifying P. pungens and P. engelmannii when the two occur in sympatry, widespread hybridization was long invoked, suggesting a close relationship (Habeck and Weaver 1969;Daubenmire 1972;Taylor et al 1975). However, the cross proved dif cult to make, and molecular data has revealed no, or few, natural hybrids (Ernst et al 1990). Because of the difculty in making controlled crosses between P. engelmannii and P. pungens, a high rate of abnormalities in the hybrid progeny, and a paucity of natural hybrids (Ernst et al 1990), P. engelmannii and P. pungens seem to be good biological species in the sense of Mayr (1963).…”

“…However, the cross proved dif cult to make, and molecular data has revealed no, or few, natural hybrids (Ernst et al 1990). Because of the difculty in making controlled crosses between P. engelmannii and P. pungens, a high rate of abnormalities in the hybrid progeny, and a paucity of natural hybrids (Ernst et al 1990), P. engelmannii and P. pungens seem to be good biological species in the sense of Mayr (1963).…”

“…Our speci c objectives were: to determine whether the genetic differences between P. engelmannii and P. mexicana were typical of the varietal level or the species level; to examine the genetic relationship of spruces from Cerro Mohinora to other populations of P. mexicana; to determine whether spruces from the Chiricahua Mountains were aligned with P. mexicana or with P. engelmannii; to determine the genetic relationship between P. martinezii and P. chihuahuana; to compare the relictual P. breweriana with the Mexican relicts; and to con rm or reject the genetic differences reported between P. engelmannii and P. pungens using a broad sample of populations, because previous studies on molecular genetic differences between the two were concentrated in a single drainage, the Dolores River, Colorado (Ernst et al 1990). We also wished to compare our results on genetics of natural populations to the classications of spruce based on morphological criteria, usually measured on herbarium specimens or on arboretum specimens planted outside their native ranges, which might lead to unknown biases.…”