Mahelka, V., Kopecký, D., Baum, B.R.
MOLECULAR BIOLOGY AND EVOLUTION
30(9):
2065-2086,
2013
Klíčová slova:
Triticeae, polyploidy, hybridization, ribosomal DNA, concerted evolution, in situ hybridization
Abstrakt:
We employed sequencing of clones and in situ hybridization (genomic and fluorescent in situ hybridization [GISH and
rDNA-FISH]) to characterize both the sequence variation and genomic organization of 45S (herein ITS1-5.8S-ITS2 region)
and 5S (5S gene + nontranscribed spacer) ribosomal DNA (rDNA) families in the allohexaploid grass Thinopyrum
intermedium. Both rDNA families are organized within several rDNA loci within all three subgenomes of the allohexaploid
species. Both families have undergone different patterns of evolution. The 45S rDNA family has evolved in a concerted
manner: internal transcribed spacer (ITS) sequences residing within the arrays of two subgenomes out of three got
homogenized toward one major ribotype, whereas the third subgenome contained a minor proportion of distinct
unhomogenized copies. Homogenization mechanisms such as unequal crossover and/or gene conversion were coupled
with the loss of certain 45S rDNA loci. Unlike in the 45S family, the data suggest that neither interlocus homogenization
among homeologous chromosomes nor locus loss occurred in 5S rDNA. Consistently with other Triticeae, the 5S rDNA
family in intermediate wheatgrass comprised two distinct array types—the long- and short-spacer unit classes. Within
the long and short units, we distinguished five and three different types, respectively, likely representing homeologous
unit classes donated by putative parental species. Although the major ITS ribotype corresponds in our phylogenetic
analysis to the E-genome species, the minor ribotype corresponds to Dasypyrum. 5S sequences suggested the contributions
from Pseudoroegneria, Dasypyrum, and Aegilops. The contribution from Aegilops to the intermediate wheatgrass’
genome is a new finding with implications in wheat improvement. We discuss rDNA evolution and potential origin of
intermediate wheatgrass.
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