Paleopolyploidy and molecular systematics of Southern African Chloridoideae

Abstract

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.

Afrikaans: Hierdie studie het gehandeloor die sistematiek van Suid-Afrikaanse verteenwoordigers van die gras subfamilie Chloridoideae. Die groep is bestudeer op molekulêre en sitogenetiese vlak. Hoofsaaklik twee basiese chromosoomgetalle word in die Chloridoideae gevind, naamlik x = 9 en 10 en is bevestig deur hierdie studie. Laasgenoemde basiese chromosoomgetal is die mees algemene en word gesien as die oorspronklike basiese chromosoomgetal waaruit ander aneuploïdiese afwykings in die groep ontstaan het. Die basiese chromosoomgetal van x = 10 is egter 'n paleopoliploïede getal aangesien eksemplare met 21) = 2x = 10 al in die subfamilie gevind is. Meeste van die chromosoomgetalle wat in die Chloridoideae gevind word het ontstaan uit die oorspronklike basiese chromosoomgetal, x = 5 of die paleopoliploïede basiese chromosoomgetal, x = 10. Poliploïdie kom algemeen voor in die grasse en die subfamilie Chloridoideae. In hierdie studie was meer as 70% van die Suid-Afrikaanse Chloridoideae poliploïed. Hierdie poliploïdie word toegeskryf aan verbastering, aangesien meeste van die bestudeerde spesies segmenteel allopoliploïed of allopoliploïed was. Dit word bewerkstelling deur 'n effektiewe ongeslagtelike voortplantingsisteem in die vorm van apomiksie. Die nukleotiedvolgordes van twee genomiese gebiede, die kern ITS en chloroplas trnL-F gebiede, is bepaal. Hierdie twee gebiede verteenwoordig verskillende genome en word verskillend oorgeërf (moederlik teenoor oorerwing vanaf albei ouers) wat filogenetiese implikasies vir die bestudering van verbastering, wat 'n algemene verskynsel in die Chloridoideae is, inhou. Die gebiede bestudeer verskil in die hoeveelheid resolusie wat hulle verskaf. Die ITS filogenie het goeie resolusie verskaf, maar die trnL-F gebied het minder variasie en swakker resolusie verskaf, veral op spesievlak. Ten spyte hiervan is geen sterk onverenigbaarheid tussen die twee filogenieë gevind .nie en kon hulle gekombineer word. Die filogenetiese analise toon 'aan dat die Chloridoideae monofileties is. Die twee groot tribusse, Cynodonteae en Eragrostideae, was polifileties, alhoewel 'n algemene skeiding in twee groepe sigbaar is. Die monofilie van al die genera in die subfamilie word goed ondersteun, behalwe die twee grootste genera in die studie, Eragrostis . en Sporobelus. Hierdie twee genera toon baie variasie en is taksonomiese moeilike groepe wat moontlik as gevolg van verbastering tussen spesies en genera is. Die morfologiese kenmerkende tribus Pappophoreae is goed ondersteun in alle analises. Die twee genera Entoplocamia en Fingerhuthia was basaal in die gekombineerde analise. Dit ondersteun die ontwikkeling van die Chloridoideae . vanaf arundinoid voorouers, aangesien hierdie twee genera gesien word as naverwant aan Spartochloa, Styppeiochloa en Tribolium in die Arundinoideae en Danthonioideae. Ongeag die groot hoeveelheid verbastering wat in die subfamilie voorkom kon basters nie met sekerheid geïdentifiseer word op grond van nukleotiedvolgorde polimorfismes of hulle filogenetiese gedrag nie. Dit hou moontlik verband met die ouderdom van die verbastering in die groep of die noue verwantskap van die groepe waarin daar verbastering voorkom. Hierdie studie lewer sitogenetiese en molekulêre sistematiese ondersteuning vir paleopoliploïdie in die Chloridoideae. Dit word grootliks gegrond op die voorkoms van x = 5 in die subfamilie en die noue verwantskap van die Chloridoideae met I Arundinoideae en Danthonioideae met 'n basiese chromosoomgetal van x = 6 en waaruit x = 5, wat in die Chloridoideae voorkom, kon ontwikkel het. Hierdie chromosoomgetal was moontlik hoogs onstabiel en poliploïdisering het gelei tot die ontstaan van x = 10 in die meeste genera en spesies in die subfamilie.