GENETIC DIFFERENTIATION DURING SPECIATION IN THE HELIANTHUS DEBILIS COMPLEX.

Abstract

The Helianthus complex of sunflowers (Asteraceae) is comprised of eight subspecies of H. debilis, predominantly from Florida and Texas (Heiser, 1956), and a closely related morphologically similar species, Helianthus petiolaris. Helianthus petiolaris and all subspecies of H. are obligate outcrossers and possess a chromosome number of n = 17 (Heiser, 1948). The investigated in this study and their geographic distributions are listed in Table 1. Heiser (1956) defines the subspecies of H. as populations or series of which replace each other geographically and at the same time show conspicuous morphological differences. The subspecies are allopatric except where the range of H. d. cucumerifolius approaches that of H. d. silvestris to the northeast, H. d. runyonii to the southeast, and H. d. hirtus to the southwest. In these areas the morphological differences between the subspecies become somewhat blurry, perhaps due to hybridization. Garden hybridization studies (Heiser, 1956) indicate the existence of two distinct breeding assemblages within H. debilis. The debilis assemblage includes H. d. debilis, H. d. silvestris, H. d. tardifiorus, H. d. vestitus, and H. d. cucumerifolius. The praecox assemblage includes H. d. praecox, H. d. runyonii, and H. d. hirtus. Crosses between subspecies within each assemblage yield highly fertile F1 hybrids with approximately 90% pollen fertility and almost 100% good seed set. Although morphological differences between the assemblages are no greater than those within each assemblage, interassemblage crosses result in F1 hybrids with pollen fertilities ranging from 20% to 50% and 33% to 50% good seed set. This reduction in fertility is most likely due to chromosomal rearrangements. The breeding groups differ by a minimum of two translocations which result in meiotic pairing abnormalities in the hybrids (Heiser, 1956). The breeding assemblages may be thought of as semispecies, with reproductive isolation intermediate to that of subspecies and species. Helianthus petiolaris replaces H. to the west and north and is distinguished by the presence of bluish-green, densely villous pale tips at the center of the infloresence disk. There is no evidence of hybridization between these allopatric species in nature. Crosses between H. petiolaris and the praecox assemblage yield F1 hybrids with approximately 40% pollen fertility, but crosses between H. petiolaris and individuals of the debilis assemblage produce highly sterile F1 hybrids with no greater than 20% pollen fertility, typical of most interspecific crosses in Helianthus (Heiser, 1956). In order to study genetic divergence during the speciation process in the Helianthus complex, six levels of increasing evolutionary divergence were examined by means of horizontal starch gel electrophoresis. Mean genetic distances among were calculated for the following levels: 1) local of each taxon; 2) subspecies within the debilis assemblage; 3) the praecox and debilis assemblages; 4) praecox and H. petiolaris; 5) debilis and H. petiolaris; and 6) H. and H. radula, a perennial more distantly related species growing in sandy soils of the pinelands of Florida, Georgia, and Alabama. Helianthus radula is morphologically very distinctive,