Chloroplast DNA evolution and the origin of amphidiploid Brassica species

Abstract

The origin and evolution of a hybrid species complex in the genus Brassica (cabbage, turnip, mustard, rapeseed oil) has been explored through mutational analysis of the maternally inherited chloroplast genome. A detailed chloroplast DNA phylogeny enables identification of the maternal parent for most of the amphidiploids examined and permits quantitative resolution of the relative time of hybridization as well as the relative divergence of the diploid parents. Contradictory chloroplast and nuclear phylogenies obtained for two accessions of the amphidiploid B. napus (rapeseed oil) lead to the hypothesis that introgressive hybridization has also figured in their recent evolution.

Chloroplast DNA evidence for the origin of the genus Heterogaura from a species of Clarkia (Onagraceae)

“…peripetasma and H. heterandra, ranged from 0.0017 to 0.0156, with an average of 0.0107 (Table 1). This value is 3-3.5 times higher than that of other cpDNAs within a genus in Brassicaceae, Gentianaceae, and Poaceae (9,11,20). The sequence divergence between H. heterandra and each of the eight Clarkia species ranged from 0.0048 (with C. dudleyana) to 0.0147 (with C. rostrata), the average being 0.0103 (Table 1).…”

“…peripetasma and H. heterandra. The highest incidence of change occurred in portions of the large single-copy region near the inverted repeat, a region where a high frequency of mutations has been noted (7,9). A single mutation was identified within the inverted repeat, confirming its high degree of conservation (6,9).…”

“…Its high degree of sequence conservation facilitates use of heterologous probes to visualize restriction fragments on Southern blots. Comparative mapping of cpDNA among congeneric species has provided detailed phylogenetic relationships in a number of Angiosperm genera (6)(7)(8)(9)(10)(11). Mapping of restriction sites in cpDNA of Heterogaura and in eight sections of Clarkia demonstrated that Heterogaura cpDNA resembled the cpDNA of species in sect.…”

Chloroplast Dna Introgression in Southern California Sunflowers

“…Despite the potential power of cpDNA variation for phylogenetic reconstruction, a number of recent studies have noted major incongruencies between cpDNA and organismal phylogenies (e.g., Palmer et al, 1983Palmer et al, , 1985Doebley, 1989;Baldwin et al, 1990;Brunsfeld, 1990;Doyle et aI., I990a. 1990b;Smith and Sytsma, 1990;Sytsma et aI., 1990;Wendel et aI., 1991;Soltis et aI., 1991;Whittemore and Schaal, 1991).…”

Molecular systematics of the genus Senecio L. I: Hybridization in a British polyploid complex

“…Even over much longer time scales the cpDNA of allopolyploids appears to be very stable (Aegilops triuncialis; Murai and Tsunewaki, 1984). However, in the case of the amphidiploid Brassica napus, the cpDNA was found to be different to the putative maternal parent, B. oleracea (Palmer et a!., 1983). Length mutations are rare events (Palmer, 1987). Due to the potential rarity of the initial mutation event and its fixation in a population in a short period of time, the most likely explanation is that one of the parental taxa carries a length mutation which has not yet been located due to the limited size of the survey.…”