The systematics and evolution of Townsendia (Compositae)

Abstract

The genus Townsendia (tribe Astereae) consists of perennial, biennial, and annual herbs that occur in the western United States, Mexico, and southwestern Canada. Twenty-one species are now recognized in the genus; two new species and one new variety have been described in this paper. Reproduction in several species of the genus has received cytogenetic and embryological study. Meiosis in both micro-and megasporogenesis in diploid, sexual plants (n = 9) is regular with the formation of tetrads of micro-and megaspores. At anthesis the pollen grains are 3-nucleate, and the percentage of aborted grains is very low. Development of the female gametophyte from the megaspore tetrad is sometimes trisporic and sometimes tetrasporic. The mature female gametophyte in sexual plants usually consists of from 12 to 16 nuclei. Apomixis has been detected in 12 species. Meiosis in both micro- and megasporogenesis in polyploid, apomictic plants (2n = 27-36) is irregular with many or all of the chromosomes occurring as univalents at metaphase I. When some bivalents are present in microsporogenesis, a tetrad usually is produced, but a wall often encloses the whole tetrad, and a giant, several-nucleate pollen grain results; at anthesis the percentage of aborted grains is high. When no bivalents are present in microsporogenesis, a restitution nucleus (chromosome clump) is formed at metaphase I; the following anaphase is equivalent to anaphase II of normal meiosis, and a dyad with unreduced nuclei results. The two halves of the dyad develop separately into 3-nucleate pollen grains that are conspicuously larger than those produced by diploid, sexual plants; at anthesis the percentage of aborted grains is high. It is suggested that the unreduced dyads resulting after apomictic megasporogenesis are produced in the same manner as those in apomictic microsporogenesis. The megaspore dyad develops directly into a 2-nucleate embryo sac, and after two series of mitotic divisions the female gametophyte in apomictic plants matures at the 8-nucleate level. Division of the unreduced egg nucleus is initiated slightly before that of the two fused polar nuclei. The female gametophyte escapes reduction by diplospory; apomixis is obligate, and pollination is not required to initiate development of the embryo or endosperm. No diploid plants are known to be apomictic; similarly, no polyploid plants are knon to be sexual. The apomicts have not been given special taxonomic recognition. They have been referred to species known also from one or more sexual populations. Apomictic plants are concentrated at the higher elevations and higher latitudes relative to the whole range of the species in which they occur. Apomixis is found most commonly in species with the perennial habit; however, it occurs also in some biennials. Hybridization apparently has been extensive in the genus. Intermediate populations involving 15 species have been found. Certain apomictic races in six species exhibit morphological evidence of hybridity, but other apomicts are clearly of non-hybrid origin. The species of Townsendia are considered to be mostly of recent derivation. The most primitive species, T. formosa, has many characters in common with three other genera, Dichaetophora, Astranihium, and Boltonia. Geographic isolation may have been the principal factor governing speciation in Townsendia. Experimental crosses and putative natural hybrids suggest that genetic isolating barriers are not involved in initial evolutionary divergence. Polyploidy apparently has not been a mechanism of evolution, as it seems to be closely associated with obligate apomixis. Several examples of parallel or convergent evolution have been discussed.