Abstract (eng)
The genus Melampodium (Asteraceae) is a group very well-suited to study the evolutionary consequences of chromosome number change and reticulation. The genus comprises a moderate number (40) of annual and perennial species classified into six sections. The genus is centered in tropical to subtropical Mexico, southwestern United States, Brazil and Colombia. Chromosome numbers and karyotypes are now known for all species except for the recently described species Melampodium moctezumum. The genus displays a wide range of haploid chromosome numbers (n = 9, 10, 11, 12, 14, 18, 20, 23, 24, 27, 28, 30, 33), one of the longest series in family Asteraceae. These haploid chromosome numbers can be allocated to five basic chromosome numbers (x = 9, 10, 11, 12, and 14). The current classification of the genus is based primarily on a combination of morphological characters and basic chromosome numbers.
The first chapter of the thesis presents a molecular phylogeny of the genus based on analyses of plastid and nuclear DNA markers, which is also used to test the current taxonomic classification. Results show that: (1) Melampodium is monophyletic if closely related genera Acanthospermum and Lecocarpus are included; (2) reticulation and hybridization events have repeatedly contributed to the evolution of Melampodium as inferred from incongruencies between plastid and nuclear phylogenies; (3) three of the six sections of the current classification are supported by both marker systems, with five out of the six sections supported by the plastid phylogeny alone; (4) section Alcina encompassing three species has been inferred as polyphyletic in both marker sets; and (5) basic chromosome numbers correlate (at least partly) with the phylogeny of the genus.
In the second chapter the directionality of the chromosome number change has been investigated using plastid matK and nuclear ribosomal ITS phylogenies of the diploid taxa and applying maximum parsimony and maximum likelihood-based reconstruction methods. All analyses support x = 11 as the most likely ancestral basic chromosome number for the genus. The basic chromosome number of x = 10, previously hypothesized to be ancestral for the genus, was reconstructed to originate once (plastid data) or twice (nuclear data). Similarly, the chromosomal base number of x = 9 has likely originated twice independently, but a single origin cannot be excluded. The chromosomal base numbers x = 12 and 14 have been shown to be derived from a common ancestor most likely based on x = 11. Descending dysploidy was shown to be more prevalent than ascending dysploidy.
The third chapter examines the hybridization events leading to the origin of six allopolyploid species of sers. Sericea and Melampodium (sect. Melampodium), and their subsequent genome evolution. A combined approach employed sequencing of plastid (matK, psbA-trnH) and nuclear DNA regions (ITS, 5SrDNA spacer, low copy PgiC gene), restriction pattern analyses of ITS, 5S and 35S rDNA mapping in the chromosomes using fluorescence in situ hybridization (FISH), and flow cytometry for genome size measurements. These allowed inferring the origins of allopolyploids and tracing genome restructuring following the polyploid establishment. Species of sers. Melampodium, Cupulata s.str., and ser. Glabribracteata were shown to be involved as parental taxa in hybridization events leading to the origin of six allopolyploid species. The genome size additivity observed in all polyploids contrasts with 35S rDNA loci loss and conversion and, albeit to lesser extent, with loss of 5S rDNA loci. Two allohexaploids, Melampodium pringlei and M. sericeum, despite originating from the same parental taxa, have been shown to follow different genome restructuring pathways.