Abstract
Hemp (Cannabis sativa L.) was karyotyped using by DAPI/C-banding staining to provide chromosome measurements, and by fluorescence in situ hybridization with probes for 45 rDNA (pTa71), 5S rDNA (pCT4.2), a subtelomeric repeat (CS-1) and the Arabidopsis telomere probes. The karyotype has 18 autosomes plus a sex chromosome pair (XX in female and XY in male plants). The autosomes are difficult to distinguish morphologically, but three pairs could be distinguished using the probes. The Y chromosome is larger than the autosomes, and carries a fully heterochromatic DAPI positive arm and CS-1 repeats only on the less intensely DAPI-stained, euchromatic arm. The X is the largest chromosome of all, and carries CS-1 subtelomeric repeats on both arms. The meiotic configuration of the sex bivalent locates a pseudoautosomal region of the Y chromosome at the end of the euchromatic CS-1-carrying arm. Our molecular cytogenetic study of the C. sativa sex chromosomes is a starting point for helping to make C. sativa a promising model to study sex chromosome evolution.
Highlights
Dioecy occurs in approximately 7% of flowering plant species [1], [2], among which only a small number of species have cytogenetic and/or molecular evidence for sex chromosomes
Cytogenetic landmarks for the sex chromosomes and fully sex-linked DNA markers have been established in species of Rumex [10], [11], [12], [13], [14], Silene [15], [16], [17], [18], [19], Humulus [20], [21], [22], [23], [24], [25], [26], and Coccinia (Cucurbitaceae) [27], but sex-linked genes have so far been identified in dioecious Silene species [28], [29], [30], [31], [32], [33] and in papaya (Carica papaya) [34]
C. sativa has a maledetermining Y [2], [35], but, in H. lupulus and H. japonicus, sex is determined by the X to autosome ratio, the Y chromosome is essential for normal pollen development in H. lupulus [36]
Summary
Dioecy occurs in approximately 7% of flowering plant species [1], [2], among which only a small number of species have cytogenetic and/or molecular evidence for sex chromosomes. C. sativa has a smaller genome size (0.84–0.91 pg [37], [38]) than those of the two Humulus species (1.7 pg for H. japonicus [39], [40], and 2.90 pg for H. lupulus [39]). The species’ estimated haploid genome sizes are 818 Mb for female plants and 843 Mb for males, indicating that the Y chromosome is larger than the X [37], this difference is not usually detectable by microscopic techniques [37], Yamada, 1943, cited by [44], [45], and very precise measurements are needed to identify the Y chromosome. Our results providing basic molecular cytogenetic data on the structure of the C. sativa genome, including the sex chromosomes, could provide a starting point for genome assembly
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