Centromeric Localization and Adaptive Evolution of an Arabidopsis Histone H3 Variant

Abstract

Centromeric H3-like histones, which replace histone H3 in the centromeric chromatin of animals and fungi, have not been reported in plants. We identified a histone H3 variant from Arabidopsis thaliana that encodes a centromere-identifying protein designated HTR12. By immunological detection, HTR12 localized at centromeres in both mitotic and meiotic cells. HTR12 signal revealed tissue-and stage-specific differences in centromere morphology, including a distended bead-like structure in interphase root tip cells. The anti-HTR12 antibody also detected spherical organelles in meiotic cells. Although the antibody does not label centromeres in the closely related species Arabidopsis arenosa , HTR12 signal was found on all centromeres in allopolyploids of these two species. Comparison of the HTR12 genes of A. thaliana and A. arenosa revealed striking adaptive evolution in the N-terminal tail of the protein, similar to the pattern seen in its counterpart in Drosophila . This finding suggests that the same evolutionary forces shape centromeric chromatin in both animals and plants. INTRODUCTIONCentromeres are the specialized chromosomal sites necessary for poleward movement during mitosis and meiosis in eukaryotes. Commonly, a centromere is evident as a prominent constriction within the heterochromatin of each metaphase chromosome. The attachment to and movement of chromosomes along the spindle is mediated by the proteinaceous kinetochores, which form at the centromeres during cell division.Despite this highly conserved function, centromeric DNA sequences are not conserved between organisms. For example, human centromeres consist of large blocks (200 kb to several megabases) of tandemly repeated 171-bp ␣ -satellite (Willard, 1998), but the sequences can differ from those of apes on homologous chromosomes (Haaf and Willard, 1997). Similarly, Drosophila melanogaster centromeric regions contain blocks of 5-to 12-bp satellite repeats that do not appear to be shared by homologous centromeres of sibling species (Lohe and Brutlag, 1987).Plant centromeric regions resemble their mammalian counterparts in that both have large arrays of tandem repeats, frequently of approximately nucleosomal size. In centromere regions of maize (Alfenito and Birchler, 1993;Ananiev et al., 1998), pearl millet (Kamm et al., 1994), rice (Dong et al., 1998), sugarcane (Nagaki et al., 1998), sorghum (Zwick et al., 2000), the Australian daisy Brachycome dichromosomatica (Leach et al., 1995), rape (Xia et al., 1993), and the wild beet Beta procumbens (Gindullis et al., 2001), tandem repeats have been found that differ in sequence but that all have lengths in the range of ‫ف‬ 140 to 180 bp. Particular repeat arrays in cereals have been estimated to be hundreds of kilobases long (Dong et al., 1998;Kaszás and Birchler, 1998). Cereal tandem repeat arrays are interrupted by TY3/gypsy -like retrotransposons and other retrotransposons (Ananiev et al., 1998;Kumekawa et al., 2001;Nonomura and Kurata, 2001). The satellite arrays and interspersed retrotransposons o...

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