Knowledge about the composition and structure ofcentromeres is critical for understanding how centromeresperform their functional roles. Here, we report the sequencesof one centromere-associated bacterial artificial chromosomeclone from a Coix lacryma-jobi library. Two Ty3/gypsy-classretrotransposons, centromeric retrotransposon of C. lacrymajobi(CRC) and peri-centromeric retrotransposon of C.lacryma-jobi, and a (peri)centromere-specific tandem repeatwith a unit length of 153 bp were identified. The CRC ishighly homologous to centromere-specific retrotransposonsreported in grass species. An 80-bp DNA region in the153-bp satellite repeat was found to be conserved tocentromeric satellite repeats from maize, rice, and pearlmillet. Fluorescence in situ hybridization showed that thethree repetitive sequences were located in (peri-)centromericregions of both C. lacryma-jobi and Coix aquatica.However, the 153-bp satellite repeat was only detected on20 out of the 30 chromosomes in C. aquatica. Immunostainingwith an antibody against rice CENH3 indicates that the153-bp satellite repeat and CRC might be both the majorcomponents for functional centromeres, but not all the153-bp satellite repeats or CRC sequences are associatedwith CENH3. The evolution of centromeric repeats of C.lacryma-jobi during the polyploidization was discussed.

The centromere of an eukaryotic chromosome can move to a newposition during evolution, which may result in a major alterationof the chromosome morphology and karyotype. This centromererepositioning phenomenon has been extensively documented inmammalian species and was implicated to play an important rolein mammalian genome evolution. Here we report a centromererepositioning event in plant species. Comparative fluorescence insitu hybridization mapping using common sets of fosmid clonesbetween two pairs of cucumber (Cucumis sativus L.) and melon(Cucumis melo L.) chromosomes revealed changes in centromerepositions during evolution. Pachytene chromosome analysis revealedthat the current centromeres of all four cucumber andmelon chromosomes are associated with distinct pericentromericheterochromatin. Interestingly, inactivation of a centromere in theoriginal centromeric region was associated with a loss or erosionof its affixed pericentromeric heterochromatin. Thus, both centromereactivation and inactivation in cucurbit species were associatedwith a gain/loss of a large amount of pericentromeric heterochromatin.

The Cucurbitaceae includes important crops such as cucumber, melon, watermelon, squash and pumpkin. However, fewgenetic and genomic resources are available for plant improvement. Some cucurbit species such as cucumber have anarrow genetic base, which impedes construction of saturated molecular linkage maps. We report herein the developmentof highly polymorphic simple sequence repeat (SSR) markers originated from whole genome shotgun sequencing and thesubsequent construction of a high-density genetic linkage map. This map includes 995 SSRs in seven linkage groups whichspans in total 573 cM, and defines, 680 recombination breakpoints with an average of 0.58 cM between two markers.These linkage groups were then assigned to seven corresponding chromosomes using fluorescent in situ hybridization(FISH). FISH assays also revealed a chromosomal inversion between Cucumis subspecies [C. sativus var. sativus L. and var.hardwickii (R.) Alef], which resulted in marker clustering on the genetic map. A quarter of the mapped markers showedrelatively high polymorphism levels among 11 inbred lines of cucumber. Among the 995 markers, 49%, 26% and 22% wereconserved in melon, watermelon and pumpkin, respectively. This map will facilitate whole genome sequencing, positionalcloning, and molecular breeding in cucumber, and enable the integration of knowledge of gene and trait in cucurbits.

The centromere of the maize (Zea mays) B chromosome contains several megabases of a B-specific repeat (ZmBs), a156-bp satellite repeat (CentC), and centromere-specific retrotransposons (CRM elements). Here, we demonstrate that onlya small fraction of the ZmBs repeats interacts with CENH3, the histone H3 variant specific to centromeres. CentC, whichmarks the CENH3-associated chromatin in maize A centromeres, is restricted to an; 700-kb domain within the largercontext of the ZmBs repeats. The breakpoints of five B centromere misdivision derivatives are mapped within this domain.In addition, the fraction of this domain remaining after misdivision correlates well with the quantity of CENH3 on thecentromere. Thus, the functional boundaries of the B centromere are mapped to a relatively small CentC-and CRM-richregion that is embedded within multimegabase arrays of the ZmBs repeat. Our results demonstrate that the amount ofCENH3 at the B centromere can be varied, but with decreasing amounts, the function of the centromere becomes impaired.