Consequences of long term artificial selection, inbreeding, and isolation in phlox: I. The evolution of cross incompatibility

The origin of genetically conditioned reproductive isolating barriers in plants has received much critical attention since this process is the essence of the speciation process. Abundant data suggest that reproductive isolating barriers are the incidental by-products of genetic or chromosomal divergence between populations (Grant, 1958, 1971; Stebbins, 1950, 1974; Levin, 1971). However, the tempo of barrier building remains a matter of conjecture and debate. The analysis of the strength of various forms of reproductive isolating mechanisms between lineages of cultivated plants whose time of divergence is known is one approach to the problem of tempo which may be used to good advantage. The present study deals with the extent to which a single barrier, cross-incompatibility, has developed within Phlox drummondii Hook. over a prescribed time period. Knowledge of horticultural practices, foundation seed source and date of collection permits us to ascertain the degree of crossincompatibility attained in ca. 140 generations of isolation, inbreeding, and divergent selection between cultivated strains of P. drummondii, and between cultivars and conspecific wild populations. The species is a self-incompatible annual and a conspicuous element of the native flora of central Texas to which it is endemic. Seeds were collected by Thomas Drummond in 1835 in areas where the red-flowered subspecies drummondii intergrades with the pink-flowered subspecies goldsmithii, which would place the collection sites south of Austin and east of San Antonio (Erbe and Turner, 1962). According to Kelly (1915), the seeds were sent to Great Britain and then distributed to nurserymen in several European countries. The nurserymen succeeded in breaching the self-incompatibility barrier, and developed true-breeding strains for multigenic characters such as corolla color ahd eye markings, corolla outlines and sizes, and plant statures (Kelly, 1920). During the 79 generations from the time P. drummondii reached Europe to 1915, over 200 true-breeding lines had been established (Kelly, 1915). Many of these cultivars are still available from European nurseries. The native phloxes now seem as polymorphic as described in accounts of Texas at the turn of the century (cf. Kelly, 1915). In summary, we have at our disposal numerous cultivars derived from a single collection of foundation seed, and the populations from which this seed was obtained. Although the precise phylogeny of the Phlox cultivars is unknown, we know that the "Grandiflora" strains were derived from the original material and were the source of "Dwarf" strains (Kelly, 1915). The relationship of the cultivar with wild P. drummondii also can be judged from their successive appearance in horticultural catalogues and from the extent to which they differ morphologically from their wild ancestor. The "Grandiflora" strains were the first to appear in catalogues, and are similar in stature to the wild Phlox but have slightly larger flowers. The "Dwarf" strains appeared several years later to be followed by the "Extra Dwarf" strains, the latter the end products of selection for low and much branched plants. The "Twinkle" strains are derived from the "Extra Dwarfs" and are characterized by a single gene mutation affecting corolla outline (Kelly, 1920). They are the most recent addition to