Abstract:
Two species of the familiar goat's beard or salsify genus Tragopogon were studied to determine the role and significance of interspecific hybridization in speciation within the genus. Field observations indicate the common occurrence of interspecific hybrids between T. pratensis, a yellow-flowered species and T. porrifolius, a species with purple flowers. The occurrence of Tragopogon interspecific hybrids could allow for one or more of the following three evolutionary developments to take-place: 1) the hybrids could develop into a new species via allopolyploidy; 2) introgressive hybridization could modify the two parental gene pools; and/or 3) a hybrid swarm having no evolutionary significance could be formed. The purpose of the present study is to determine whether interspecific hybridization between T. pratensis and T. porrifolius is of major significance in the evolution of genus Tragopogon.Of the four basic criteria (isolation, time, mutation and adaptation) required to produce evolutionary change, the presence or absence of isolation is the focus of the present investigation. Five evidences of isolation were studied: (1) A comparison was made for the three Tragopogon taxa between the incidence of seed germination when inflorescences were covered prior to anthesis versus when they were left uncovered. (2) Pollen viability counts were made to determine the incidence of viable microspores produced by plants of the three taxa. (3) Cytologic studies of microsporogenesis in the two parental species and their interspecific hybrid were made to compare chromosome homologies by determining whether synapsis was regular in the spore mother cells. (4) Paper chromatographic analysis of amino acids and secondary substances (flavonoids) was performed to see if chemical similarities and dissimilarities exist among the Tragopogon taxa studied. (5) Finally, insect pollinators which frequent the three Tragopogon taxa were collected and identified to investigate whether there is a commonality of insects visiting the Tragopogon flowers.The data indicate that the three taxa have a lower seed set and lower germination rate when the flower heads are covered prior to opening than when the flower heads are left undisturbed so that open pollination occurs. The results from seed germination rates indicate that the seeds derived from open pollinated inflorescences sprouted more quickly than did seeds from flowers which were bagged prior to anthesis. The pollen viability studies indicate that T. porrifolius had the highest level of pollen viability--94%; T. pratensis ranked second with 90% viable pollen; and the hybrid had the least viable pollen--19%. Microscopic examination of PMC's undergoing meiosis to produce microspores reveals a somatic chromosome count of 2n = 12 for each of the three taxa studied. Synapsis during prophase appears normal in both the parental species and the interspecific hybrids. Single dimensional paper chromatography reveals a chemical similarity between T. pratensis and the hybrid for the amino acid distributions. T. porrifolius displayed a dissimilar amino acid pattern when compared with the other two taxa. Two dimensional paper chromatograms of secondary substances indicate that the Tragopogon interspecific hybrids contained a blend of the flavonoids which were found in the two parental species. Many insects are found feeding or resting upon the inflorescences of the three taxa. Most of the insects are dipterans of the family Syrphidae. Many of the same insect genera are found on the flowers of the three Tragopogon taxa.Evidence from the current study does not suggest that a new Tragopogon species has formed or is forming. Before a diploid interspecific hybrid could diverge, genetically, from its parental ties there must be reproductive isolation coupled with hybrid fertility. These two criteria were not met; no fertile allotetraploid hybrid derivatives were identified. So, a Tragopogon micton or alloploidion is not forming.The two parental Tragopogon species are homogameons. There is no indication of introgression taking place. The highly infertile F1 interspecific hybrids do not appear to be cross-pollinating with the two parental species. This lack of introgression may be explained by the low incidence of viable pollen grains from the hybrid plants within the populations.Many highly sterile interspecific Tragopogon hybrids are formed year after year and there is no indication of genetic mingling with the parental populations. This situation indicates that a hybrid swarm is present which has no evolutionary significance.