Cytogenetic events in the endosperm of amphiploid Avena magna × A. longiglumis

Abstract

This study analysed cytogenetic events occurring in the syncytial endosperm of the Avena magna H. C. Murphy & Terrell × Avena longiglumis Durieu amphiploid, which is a product of two wild species having different genomes. Selection through the elimination of chromosomes and their fragments, including those translocated, decreased the level of ploidy in the endosperm below the expected 3n, leading to the modal number close to 2n. During intergenomic translocations, fragments of the heterochromatin-rich C-genome were transferred to the D and Al genomes. Terminal and non-reciprocal exchanges dominated, whereas other types of translocations, including microexchanges, were less common. Using two probes and by counterstaining with DAPI, the A. longiglumis and the rare exchanges between the D and Al genomes were detected by GISH. The large discontinuity in the probe labelling in the C chromosomes demonstrated inequality in the distribution of repetitive sequences along the chromosome and probable intragenomic rearrangements. In the nucleus, the spatial arrangement of genomes was non-random and showed a sectorial-concentric pattern, which can vary during the cell cycle, especially in the less stable tissue like the hybrid endosperm.

Keywords: Amphiploid; Avena; Chromosome rearrangements; Genome domains; Nuclear disorders.

Fig. 1

Examples of cytogenetic disorders in: A. magna (a–c), A. longiglumis (d–f) and A. magna × A. longiglumis amphiploid (g and h). a Metaphase with a fragment of chromosome forming a micronucleus (arrow), b anaphase with multiple (ca. three) bridges (arrowhead) and two precocious chromosome units at the poles (arrows) which are still connected to the poles, c nucleus with two micronuclei (arrows), d an irregular nucleus (arrow) before the creation of two nuclei of different sizes, e nuclei with micronuclei (arrows) and a highly condensed nucleus extruding a micronucleus of the same kind (dot-arrows), f anaphase with several bridges (arrows), g a micronucleus (arrow) and a group of nuclei at the early prophase, h – a micronucleus composed of two genomes, red C genome (arrow) and green A genome (dot-arrow). DAPI fluorescence for a–g and counterstained by propidium iodide for a, b, c, e, g. For h red fluorescence for A. eriantha probe (CpCp genomes) and green fluorescence for A. nuda probe (AsAs genomes). Scale bars 10 µm

Fig. 2

Chromosome rearrangements in A. magna detected by GISH. a Prophase with four D/C (dot-arrows) and six C/D translocations (arrows for terminal, arrowheads for intercalary), 1 and 1’ a pair of terminal reciprocal microtranslocations; b green chromosomes of the D genome marked with the A. nuda probe (dot-arrows) and DAPI blue chromosomes of the C genome (arrows), c anaphase translocations (small arrows) and a bridge (large arrow); in both groups of chromosomes, the same number of translocations is marked by arrows for terminal C/D, arrowheads for subterminal C/D and dot-arrows for D/C. Scale bars 10 µm

Fig. 3

Detection of genomes and translocations in the Am × Al free-nuclear endosperm. Two probes of the total genomic DNA were used—Cp genome from A. eriantha (red) and As genome from A. nuda (green); in addition, the slides were counterstained by DAPI (blue). a A metaphase (3n = 42) showing DAPI fluorescence, with different sectors in chromosomes separated by dotted lines, numbers and letters; (1 and 2 mark the sectors of lower DNA condensation, and for others, designation of sectors with reduced homology to the probes used (see b–f); b red fluorescence of eight terminal (arrows) and one intercalary (arrowhead) C/D, Al translocations marked by using the A. eriantha probe, with white dots indicating the C genome chromosomes; c green fluorescence shows the homology of three genomes (C, D, Al) of the amphiploid after hybridisation of the A. nuda probe; d red and green fluorescence of two overlapped probes marks the same translocations as in b, and in addition one tiny terminal D/C translocation (dot-arrow); for chromosomes ‘a’ to ‘e’, the sectors of lowered homology to the C genome probe or even arm translocations (see ‘c’ chromosome) are shown; e the metaphase marked by DAPI and green fluorescence of the A. eriantha probe – three genomes can be identified: white dots for C, black for D and grey for Al genome; for chromosomes ‘1’ and’2’ arms of lower DNA condensation and for chromosome ‘3’ the possible D/Al arm translocation (also shown in f) are distinguished; f the metaphase with the DAPI fluorescence and fluorescence of both probes overlapped (for chromosomes ‘a’ and, ‘e’ intercalary and for chromosomes ‘b, ‘c’ and ‘d’ large terminal lowering of homology to the Cp probe or chromosome rearrangements are well visible). Scale bars 10 µm

Fig. 4

Spatial arrangement of genomes in A. magna (a) and the amphiploid (b–d). a A prophase sectorial-concentric distribution of the D (lines) and C (dashed lines) genome chromosomes, with terminal (arrows) and intercalary (arrowhead) translocations found outside; b an interphase-prophase nucleus with a condensed centre (dot-arrow), decondensedexternal parts (arrows), the C genome condensed at the periphery of the nucleus (white dots) and an orange dot showing the D/C chromosome microrearrangement (small arrow with dashed line); c a side-by-side arrangement of the green D, Al genome (a) and red C genome (b) in the nucleus of an early prophase; d a sectorial-concentric arrangement of genomes in the nucleus of prophase (green D genome (arrows), red C genome (white dots) and blue Al genome (asterisks)). Scale bars 10 µm