Paleopolyploidy and molecular systematics of Southern African Chloridoideae

[摘要] English: This study dealt with systematics of southern African representatives of thegrass subfamily Chloridoideae. The group was studied on molecular andcytogenetic 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 isthe most prevalent and is seen as the original number from which other aneuploiddeviations in the group arose. The basic chromosome number of x = 10 is,however, a paleopolyploid number as specimens with 2n == 2x = 10 have been foundin the subfamily. Most of the chromosome numbers found in the Chloridoideae arederived from the original basic chromosome number, x = 5, or the paleopolyploidnumber, x = 10.Polyploidy is frequent in the grasses and subfamily Chloridoideae. In thisstudy more than 70% of the southern African Chloridoideae was found to bepolyploid. This polyploidy is largely attributed to hybridization, as many of thestudied specimens were segmental allopolyploids or allopolyploids. This isfacilitated by an effective asexual reproduction system in the form of apomixis.Two genomic regions were sequenced in this study, i.e. the nuclear ITS andchloroplast trnL-F regions. These two regions represent two different genomes andare inherited differently (maternal versus biparental), -which have phylogeneticimplicationsfor studying hybridization, a frequent phenomenon in the Chloridoideae.The regions studied differed in the amount of resolution they provided. TheITS phylogeny was well resolved, but the trnL-F region had less variation and lessresolution, especially at species level. Despite this no hard incongruence was foundbetween the two phylogenies and they could be combined.The phylogenetié analyses indicated the monophyletic nature of theChloridoideae. The two large -tribes, Cynodonteae and Eragrostideae werepolyphyletic, although a general division into two separate groups was evident. Themonophyly of all the generic groups in the subfamily was well supported, except forthe two largest genera in the study, Eragrostis and Sporobolus. These two generaare very variable and taxonomically difficult groups, probably related to interspecificand -generic hybridization.The morphologically distinct tribe Pappophoieae was well supported in allanalyses. The two genera Entoplocamia and Fingerhuthia was found basal in thecombined analysis, a finding that supports the derivation of the Chloridoideae fromarundinoid 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 bepositively identified based on sequence polymorph isms or their phylogenetic·behavior. This is possibly related to the age of hybridization in the group or theclose relationship of the groups ~etween which hybridization occurs. 'This study provides cytogenetic and molecular systematic support forpaleopolyploidy in the Chloridoideae. This is based mainly on the occurrence of x =5 in the subfamily and the close relationship of the Chloridoideae to theArundinoideae and Danthonioideae which have a main basic chromosome numberof x = 6 and from which x = 5 in the Chloridoideae was derived. This chromosomenumber was probably highly unstable and subsequent polyploidization lead to thenow frequent x = 10 found in the majority of the subfamily.

[发布机构] University of the Free State