Abstract
LTR-retrotransposons, knobs and structural chromosome alterations contribute to shape the structure and organization of the Zea mays karyotype. Our initial nuclear DNA content data of Z. mays accessions revealed an intraspecific variation (2 C = 2.00 pg to 2 C = 6.10 pg), suggesting differences in their karyotypes. We aimed to compare the karyotypes of three Z. mays accessions in search of the differences and similarities among them. Karyotype divergences were demonstrated among the accessions, despite their common chromosome number (2n = 20) and ancestral origin. Cytogenomic analyses showed that repetitive sequences and structural chromosome alterations play a significant role in promoting intraspecific nuclear DNA content variation. In addition, heterozygous terminal deletion in chromosome 3 was pointed out as a cause of lower nuclear 2 C value. Besides this, translocation was also observed in the short arm of chromosome 1. Differently, higher 2 C value was associated with the more abundant distribution of LTR-retrotransposons from the family Grande in the karyotype. Moreover, heteromorphism involving the number and position of the 180-bp knob sequence was found among the accessions. Taken together, we provide insights on the pivotal role played by repetitive sequences and structural chromosome alterations in shaping the karyotype of Z. mays.
Highlights
The genus Zea is a group of annual and perennial grasses native to a region extending from Mexico to Central America[1]
Transposable elements (TEs) constitute over 85% of the maize reference (B73) genome[4]; of these, ~70% belong to Class-I long terminal repeat (LTR) retrotransposons, which replicate through an RNA intermediate, as in the superfamilies Gypsy and Copia[8]
Considering the remarkable karyotype dynamism, intraspecific variation in nuclear genome size and chromosomal DNA15,36 within Z. mays, the aim of this study was to perform a comparative analysis of the karyotypes of different Zea accessions, seeking to identify if the nuclear genome size variation among them is promoted by differential amounts of repetitive sequences and/or by structural chromosomal rearrangements
Summary
The genus Zea is a group of annual and perennial grasses native to a region extending from Mexico to Central America[1]. The families Huck, Cinful, Tekay/Prem-1 and Grande belong to the superfamily Gypsy, while Prem-2/Ji and Opie are included in Copia[8,9] These families constitute a large fraction of the Z. mays genome and are distributed throughout its ten chromosomes[10], but predominantly mapped in heterochromatic regions[9]. Despite the recognized role of TEs and knobs on the dynamism of Z. mays genome size variation, little is known about the important of these sequences on plant fitness. Considering that knob heteromorphism correlates with nuclear genome size, it is fundamental to map these portions in Z. mays chromosomes in order to verify their involvement in DNA content divergence
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