Abstract
Maize karyotype variability has been extensively investigated. The identification of maize somatic and pachytene chromosomes has improved with the development of fluorescence in situ hybridization (FISH) using tandemly repeated DNA sequences as probes. We identified the somatic chromosomes of sister inbred lines that were derived from a tropical flint maize population (Jac Duro [JD]), and hybrids between them, using FISH probes for the 180-bp knob repeat, centromeric satellite (CentC), centromeric satellite 4 (Cent4), subtelomeric clone 4-12-1, 5S ribosomal DNA and nucleolus organizing region DNA sequences. The observations were integrated with data based on C-banded mitotic metaphases and conventional analysis of pachytene chromosomes. Heterochromatic knobs visible at pachynema were coincident with C-bands and 180-bp FISH signals on somatic chromosomes, and most of them were large. Variation in the presence of some knobs was observed among lines. Small 180-bp knob signals were invariant on the short arms of chromosomes 1, 6, and 9. The subtelomeric 4-12-1 signal was also invariant and useful for identifying some chromosomes. The centromere location of chromosomes 2 and 4 differed from previous reports on standard maize lines. Somatic chromosomes of a JD line and the commonly used KYS line were compared by FISH in a hybrid of these lines. The pairing behavior of chromosomes 2 and 4 at pachytene stage in this hybrid was investigated using FISH with chromosome-specific probes. The homologues were fully synapsed, including the 5S rDNA and CentC sites on chromosome 2, and Cent4 and subtelomeric 4-12-1 sites on chromosome 4. This suggests that homologous chromosomes could pair through differential degrees of chromatin packaging in homologous arms differing in size. The results contribute to current knowledge of maize global diversity and also raise questions concerning the meiotic pairing of homologous chromosomes possibly differing in their amounts of repetitive DNA.
Highlights
The identification of chromosomal features and the characterization of the maize genome structure have progressed extensively (Anderson et al, 2004; Kato et al, 2004; Schnable et al, 2009; Figueroa and Bass, 2012; Ghaffari et al, 2013), since the development of procedures for identifying maize meiotic chromosomes in the 20th century (McClintock, 1930; Longley, 1939; Rhoades, 1950)
Chromosomes 2 and 4 were recognized in the C-banded metaphases according to fluorescence in situ hybridization (FISH) data using the chromosome-specific 5S ribosomal DNA (5S rDNA) probe for chromosome 2 (Mascia et al, 1981) and centromeric satellite 4 (Cent4) for chromosome 4 (Page et al, 2001), as seen in the metaphase from a 441123 × 442612 hybrid (Figure 1D)
Chromosome 6 was identified by the nucleolus organizing region (NOR) rDNA signal (Figure 1E), and by the secondary constriction and satellite on 6S which were visualized in the C-banding and FISH spreads (Figures 1 and 4)
Summary
The identification of chromosomal features and the characterization of the maize genome structure have progressed extensively (Anderson et al, 2004; Kato et al, 2004; Schnable et al, 2009; Figueroa and Bass, 2012; Ghaffari et al, 2013), since the development of procedures for identifying maize meiotic chromosomes in the 20th century (McClintock, 1930; Longley, 1939; Rhoades, 1950). The construction of the earliest detailed meiotic cytogenetic maps (Neuffer et al, 1997) involved the location of heterochromatic regions, such as cytologically observable knobs and centromeric heterochromatin, in addition to the identification of chromosomes by relative length and arm ratio. Meiotic chromosome analysis continues to be an important tool for maize cytogenetics; on the other hand, procedures for the identification of maize mitotic chromosomes in root tip spreads have been developed. The examination of C-banded somatic metaphases has allowed the detection of bands corresponding with knobs visualized on meiotic chromosomes (Aguiar-Perecin and Vosa, 1985) and proven to be useful for the detection of homozygous and heterozygous knobs in many individuals www.frontiersin.org
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