A Cytogenetic Study of Clarkia Unguiculata. I. Translocations

Catastrophic Selection as a Factor in Speciation

“…Despite the extensive genome reorganizations that characterize different species, chromosome arrangements within a species appear to be remarkably stable and several species are apparently chromosomally homorphic. Even the two chromosomally most variable and most widely occurring diploid species, C. amoena and C. unguiculata, are each characterized by one arrangement that is most abundant throughout all or nearly all of the area of distribution (Hakansson, 1942;Mooring, 1958). In other words there is little tendency in this genus to form chromosome races within species, notable exceptions being the two chromosome races of C. speciosa (Lewis, 1959) and the chromosomal differentiation in C. dudleyana (Snow, 1960).…”

CHROMOSOMAL REPATTERNING INCLARKIA RHOMBOIDEAAS EVIDENCE FOR POST-PLEISTOCENE CHANGES IN DISTRIBUTION

“…Reciprocal translocations which result in the rearrangement of the sequence of chromosome arms are a characteristic feature in most species of Clarkia Lewis, 1953, 1955;Vasek, 1958;Mooring, 1958;Snow, 1960). These translocations usually involve most of the chromosome arm and generally result in the formation of alternately segregating rings at meiosis.…”

RAPID EVOLUTION IN CLARKIA

“…Not only are there a number of chromosomally homo-morphic species, such as C. rublcunda, occupying a variety of habitats, but even those species, such as C. amoena and C. unguiculata, which are chromosomally extremely polymorphic, have one arrangement that is characteristic of the species and occurs in high frequency throughout its range, except perhaps for occasional colonies (Hakansson, 1946;Mooring, 1958).…”

“…Like the latter pair of species, C. exilis and C. unguiculata have the same chromosome number. Clarkia unguiculata is chromosomally polymorphic (Lewis, 1951;Mooring, 1958) and to some extent so is C. exilis (Vasek, 1958). The translocation heterozygotes in both of these species, however, regularly form catenations that include all of the chromosomes involved and disjunction is regular.…”