Intergeneric hybrids in the lychnideae (Caryophyllaceae)

Debate on the objectivity of the genus rarely has reached the intensity of that generated over the category of species. Taxonomists-alpha and omega, plant and animal-have nearly all agreed that the genus is a category of convenience where generic limits are blurred (Mayr 1942, Stebbins 1950). Where extinction, isolation, or adaptive specialization has resulted in an abrupt and sharp discontinuity, the genus becomes a focal node in phylogeny and may be defined with a greater degree of objectivity. Clearly then, the degree of evolutionary divergence between related generic phylads will determine their distinctness. The tribe Lychnideae of the family Caryophyllaceae binds together a number of generic tendencies, some of which have achieved undisputed taxonomic status, while others continue to undergo reapportionment decade after decade. Drypsis, Cucubalus, Petrocopttis, and Agrostemma are lineages of such undisputed generic status; all are small genera, each with its own distinct and divergent pattern. The remaining genera of the tribe (Silene, Heliosperma, Melandrium, Lychnis, and VTiscaria) are the troublesome ones; they have defied clearcut delineation for many years. Certain members of the three largest generaSilene (ca. 500 species), Lychnis (ca. 10 species), and Melandrium (ca. 60 species)-have slipped in and out of these three repeatedly. What follows in this paper concerns results of intentional crosses made among various species usually placed in Silene, Lychnis, or Melandriu,m. The results of interspecific hybridization often lead to the answering of questions regarding genetic relationship among species. The technique of deliberate hybridizing yields data on crossability, hybrid fertility, and the nature of genetic control of phenotype, all of which are useful in assessing the degree of relationship between species. For the past several years, I have carried on such a program, mainly involving the North American species of Silene (Kruckeberg 1955, 1961). A number of taxa traditionally treated as species of Lychnis and Melandrium were also included in the crossing program. Intergeneric hybridizations pose several questions. Are crosses between species of different genera significantly different from those between species of the same genus? In other words, are we justified in expecting that crossability and hybrid fertility will be lower in the intergeneric hybrids? If there is a significant difference between intraand intergeneric crosses for these criteria, then evidence from crossings would reinforce the taxonomic judgments of separate generic status for the species in question. We would also want to know how the "key" characters traditionally used to separate the genera fare phenotypically in the hybrids. In a word, are the key morphological differences separating the genera oligogenically controlled (as simple Mendelian traits) or are they under polygenic control? The plants used for hybridizing were either collected in the wild or were obtained from botanical gardens and private collectors. Table 1 gives the origins and chromosome. numbers of theplants used; seventeen taxa are represented.