Paleopolyploidy and molecular systematics of Southern African Chloridoideae

English: This study dealt with systematics of southern African representatives of the grass subfamily Chloridoideae. The group was studied on molecular and cytogenetic levels. Two main basic chromosome numbers in the Chloridoideae, namely x = 9 . and 10, were confirmed by this study. The basic chromosome number of x = 10 is the most prevalent and is seen as the original number from which other aneuploid deviations in the group arose. The basic chromosome number of x = 10 is, however, a paleopolyploid number as specimens with 2n == 2x = 10 have been found in the subfamily. Most of the chromosome numbers found in the Chloridoideae are derived from the original basic chromosome number, x = 5, or the paleopolyploid number, x = 10. Polyploidy is frequent in the grasses and subfamily Chloridoideae. In this study more than 70% of the southern African Chloridoideae was found to be polyploid. This polyploidy is largely attributed to hybridization, as many of the studied specimens were segmental allopolyploids or allopolyploids. This is facilitated by an effective asexual reproduction system in the form of apomixis. Two genomic regions were sequenced in this study, i.e. the nuclear ITS and chloroplast trnL-F regions. These two regions represent two different genomes and are inherited differently (maternal versus biparental), -which have phylogeneticimplications for studying hybridization, a frequent phenomenon in the Chloridoideae. The regions studied differed in the amount of resolution they provided. The ITS phylogeny was well resolved, but the trnL-F region had less variation and less resolution, especially at species level. Despite this no hard incongruence was found between the two phylogenies and they could be combined. The phylogenetié analyses indicated the monophyletic nature of the Chloridoideae. The two large -tribes, Cynodonteae and Eragrostideae were polyphyletic, although a general division into two separate groups was evident. The monophyly of all the generic groups in the subfamily was well supported, except for the two largest genera in the study, Eragrostis and Sporobolus. These two genera are very variable and taxonomically difficult groups, probably related to interspecific and -generic hybridization. The morphologically distinct tribe Pappophoi"eae was well supported in all analyses. The two genera Entoplocamia and Fingerhuthia was found basal in the combined analysis, a finding that supports the derivation of the Chloridoideae from arundinoid ancestors as these two genera are seen as a link to Spartochloa, Styppeiochloa and Tnbolium in the Arundinoideae and Danthonioideae. Despite the frequency of hybridization in the subfamily, hybrids could not be positively identified based on sequence polymorph isms or their phylogenetic· behavior. This is possibly related to the age of hybridization in the group or the close relationship of the groups ~etween which hybridization occurs. ' This study provides cytogenetic and molecular systematic support for paleopolyploidy in the Chloridoideae. This is based mainly on the occurrence of x = 5 in the subfamily and the close relationship of the Chloridoideae to the Arundinoideae and Danthonioideae which have a main basic chromosome number of x = 6 and from which x = 5 in the Chloridoideae was derived. This chromosome number was probably highly unstable and subsequent polyploidization lead to the now frequent x = 10 found in the majority of the subfamily.