With 900 species, native to every continent except Antarctica and Australia (Mabberly, 2008) and possessing a myriad of growth habits and flower colors, the genus Salvia represents an extraordinary range of diversity. Several species are widely used as ornamental plants. Salvia splendens and S. farinacea are widely grown seed-propagated annual bedding plants, whereas S. nemorosa is a popular herbaceous perennial species with both seed and vegetatively propagated cultivars. Although these and other species of Salvia are widely used horticulture crops, the diversity of the genus represents a largely untapped resource for crop improvement, which might be accessed by means of interspecific hybridization. There are a number of known interspecific hybrids in the genus. However, with the exception of intentional crosses between culinary sage (S. officinalis) and S. lavandulifolia (Sanchez-Gomez et al., 1995), S. fruticosa, and S. tomentosa (Putievsky et al., 1990), all reported hybrids have occurred spontaneously either in the wild or in cultivation. There is a paucity of information concerning the interspecific fertility of Salvia species.
The crossability of two species is often related to phylogenetic relationship and chromosome number. Crosses between closely related species with the same chromosome number are more likely to hybridize and produce fertile progeny, whereas crosses between distantly related species with different chromosome numbers are more likely to either not hybridize or to produce sterile offspring resulting from genetic incongruity or meiotic errors (Harlan and DeWet, 1971; Sybenga, 1992).
The phylogenetic grouping of species into the genus Salvia was based on a key morphological feature, namely the elongated connective between the two thecae of the anthers (Claßen-Bockhoff et al., 2003). The validity of this grouping was called into question by Walker et al. (2004), who constructed a phylogeny based on rbcL and trnL-F gene sequences from 127 Salvia taxa. Based on these data, they concluded that Salvia is not monophyletic, but rather composed of three separate monophyletic clades. Although the polyphyletic nature of the genus was unexpected, the clades they described largely corresponded with the most widely accepted infrageneric grouping of the genus based on Bentham (1848, 1876). Clades 1 and 3 comprise all European, Asian, and African Salvia species and one small, morphologically distinct group of American species belonging to Clade 1. Clade 1 is primarily native to Europe and includes members of subgenera Salvia, Leoinia, and Sclarea, whereas Clade 3 is largely East Asian and corresponds generally to section Drymospace. Clade 2 is comprised of species that are exclusively native to the Americas and is comprised of the subgenera Audibertia and Calosphace. Because the clades defined by Walker correspond largely with geographical distribution and with the subgenera and sections of Bentham, it is possible to extrapolate from the species included in Walker's study to classify most other Salvia species as probable members of one of the three clades. The wide divergence of these clades, especially Clades 1 and 2, lead to the hypothesis that crosses between the different clades will not be successful.
The chromosome numbers of Salvia species are unusual in their extreme variability. Published counts range from a low of 2n = 12 in S. hispanica (Harley and Heywood, 1992) to a high of 2n = 88 in the octoploid S. guaranitica (Alberto et al., 2003). In addition to wide variation in ploidy level in Salvia, the basic number of chromosomes is also wide-ranging with x reported as 6, 7, 8, 9, 10, 11, and 15 for species within the genus (Goldblatt and Johnson, 1979; Harley and Heywood, 1992). Seven of 13 species studied by Alberto et al. (2003) had at least one known supernumerary chromosome (non-essential chromosomes absent in some individuals). It is possible that closely related species that appear to have different basic chromosome numbers might simply have supernumerary chromosomes that have not been properly identified as such.
To evaluate the potential of selected Salvia species as genetic sources for improvement of cultivated Salvia species and to provide information for plant breeders interested in developing hybrid Salvia, our objectives were to: 1) test the interspecific crossability of a select number of Salvia species from different clades and with different chromosome numbers; 2) establish chromosome numbers for selected Salvia species either lacking published counts or with inconsistently reported counts; and 3) evaluate inheritance of crop quality traits in the interspecific hybrid S. nemerosa × S. transsylvanica.
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