PIRES, J. CHRIS1*, ANNA M. SHERWOOD2, K. YOONG LIM3, ANDREW R. LEITCH3, ILIA J. LEITCH1, MICHAEL D. BENNETT1, PAMELA S. SOLTIS4, and DOUGLAS E. SOLTIS4. 1Royal Botanic Gardens Kew, Richmond, Surrey TW9 3DS, U.K.; 2School of Biological Sciences, Washington State University, Pullman WA 99164, U.S.A.; 3School of Biological Sciences, Queen Mary, University of London, London E1 4NS, U.K.; 4Genetics Institute, University of Florida, Gainseville FL 32611, U.S.A.. - Integrating molecular cytogenetics and phylogenetics: genome evolution in diploid and polyploid Tragopogon (Asteraceae).
Recent mapping and chromosome painting studies of polyploid crops
indicate that extensive genomic restructuring can occur following
polyploidization. For example, different patterns of rDNA evolution
are found in crop allopolyploids: interlocus homogenization (gene
conversion) of rDNA loci can occur in Nicotiana and
Gossypium, but is not reported in Brassica and
Triticale. However, while synthetic polyploids of crop plants
have been used to demonstrate rapid genome evolution, a naturally
occurring diploid-allopolyploid system still needs to be well
characterized. Tragopogon provides unique opportunities to
investigate recent, recurrent allopolyploidy. Previous molecular
studies indicate that T. mirus may have evolved 12 times and
T. miscellus may have formed 20 times within the past 70 years.
We are evaluating the two allopolyploids, T. mirus and T.
miscellus, relative to their three diploid progenitors using
molecular cytogenetic and phylogenetic approaches. Fluorescent in
situ hybridization (FISH) is being used to identify several loci
on individual chromosomes to detect chromosomal evolution. Probes for
the 18S-5.8S-26S array, 5S array, and other repeats are being
evaluated in diploids and recurrent polyploids for number of loci and
activation. Nucleolar dominance is a common feature in many
allopolyploids, presumably only when the units have not homogenized
and remain clearly related to the diploid progenitors. Thus, the rDNA
unit structure data predicts that nucleolar dominance may be a feature
of Tragopogon allopolyploids. Also, phylogenetic studies of
nrDNA show perfect additivity (no interlocus homogenization) in that
all cloned ITS sequences from Tragopogon polyploids give
sequences from the progenitor diploids. However, Southern
hybridization to the diploids reveals that all units of the 18S units
of an array are similar, yet there are clearly population differences
amongst the diploids, showing that intralocus gene conversion
(homogenizing the entire array) must be occurring. Integrating
molecular cytogenetics and phylogenetics is proving a powerful
approach in assessing the complex histories of these dynamic polyploid
genomes.
Key words: genome evolution, molecular cytogenetics, phylogenetics, polyploidy, rDNA gene conversion, Tragopogon