Chromosome evolution in marginal populations of Aegilops speltoides: causes and consequences

Abstract

Background: Genome restructuring is an ongoing process in natural plant populations. The influence of environmental changes on the genome is crucial, especially during periods of extreme climatic fluctuations. Interactions between the environment and the organism manifest to the greatest extent at the limits of the species' ecological niche. Thus, marginal populations are expected to exhibit lower genetic diversity and higher genetic differentiation than central populations, and some models assume that marginal populations play an important role in the maintenance and generation of biological diversity.

Scope: In this review, long-term data on the cytogenetic characteristics of diploid Aegilops speltoides Tauch populations are summarized and discussed. This species is distributed in and around the Fertile Crescent and is proposed to be the wild progenitor of a number of diploid and polyploid wheat species. In marginal populations of Ae. speltoides, numerical chromosomal aberrations, spontaneous aneuploidy, B-chromosomes, rDNA cluster repatterning and reduction in the species-specific and tribe-specific tandem repeats have been detected. Significant changes were observed and occurred in parallel with changes in plant morphology and physiology.

Conclusions: Considerable genomic variation at the chromosomal level was found in the marginal populations of Ae. speltoides. It is likely that a specific combination of gene mutations and chromosomal repatterning has produced the evolutionary trend in each specific case, i.e. for a particular species or group of related species in a given period of time and in a certain habitat. The appearance of a new chromosomal pattern is considered an important factor in promoting the emergence of interbreeding barriers.

Fig. 1.

Morphological characteristics of Ae. speltoides. (A) Left, normal spike from Ae. speltoides ssp. ligustica; right, normal spike from Ae. speltoides ssp. aucheri; centre, abnormal intermediate phenotype. (B) The atypical development of a secondary tiller from the lateral bud (arrowed) on the main culm.

Fig. 2.

Fluorescence in situ hybridization (FISH) on somatic chromosomes of Ae. speltoides with Spelt 1 (green), Spelt 52 (red), As5SDNAE (5S rDNA, pink pseudocolour) and pTa71 (45S rDNA, blue pseudocolour) DNA probes, and differential staining with 4′,6-diamidino-2-phenylindole (DAPI). (A) Original triploid genotype from the Ramat Hanadiv population; one B-chromosome also appears. (B) Diploid genotype with three Bs from the Ramat Hanadiv population. Inset B-chromosomes: left, large intercalary Ty3-gypsy cluster (Belyayev et al., 2001) in the long arm; right, intercalary Spelt 1 and 5S rDNA clusters in the long arm and a distal 5S rDNA cluster in the short arm. (C) Metaphase plate of the plant from the marginal Cankiri population: loss of almost all terminal Spelt 1 and a reduced number of Spelt 52 clusters in comparison with the Ramat Hanadiv population. Inset: chromosome 4 contains intercalary and near-centromeric Spelt 1 clusters.

Fig. 3.

Fluorescence in situ hybridization (FISH) on meiotic chromosomes of Ae. speltoides from the Kishon population with As5SDNAE (5S rDNA, red), pTa71 (45S rDNA, green), CCS-1 [cereal centromere sequence, green (Aragon-Alcaide et al., 1996)]. (A) Heterologous synapses between the long arms of chromosomes 5 and 1 (white arrow) and between B- and A-chromosomes (yellow arrow). (B) A synapse between A- and B-chromosomes is shown with a yellow arrow; chromosome 5 is heterozygous for pericentric inversion and carries an intercalary additional 5S rDNA cluster (white arrow). (C) Left, two additional 5S rDNA clusters are marked with arrows; right, a cell-specific additional 45S rDNA intercalary cluster is marked with an arrow.

Fig. 4.

Chromosomal pattern of 5S rDNA (red) and 45S rDNA (green) of Ae. speltoides and Ae. sharonensis from the Kishon populations (Raskina et al., 2004b). Modified genotypes of both species carry additional rDNA clusters on the chromosomes 1, 5 and 6.