| In this study, homologous degree and distribution characteristics of repetitive DNA sequences were analyzed on the chromosomes of maize and its close relatives. The single copy genes mir1 and dad1 were also physically mapped onto the chromosomes of maize and its close relatives. A new hexaploid cyto-type of Coix aquatica was identified and its origin was analyzed. The main results are as follows:1. Maize belongs to genus Zea of tribe Maydeae in Gramineae. Genera Tripsacum and Coix are the close relatives of the genus Zea in the tribe Maydeae. Genomes of maize and its close relatives share their repetitive DNA sequences with a large proportion. Homologous degree of repetitive DNA sequences among different species reveals their evolutionary relationship in a certain degree. A new molecular banding technique has been developed through genomic in situ hybridization (GISH). In the GISH, a given genomic DNA was hybridized to chromosomes of their own or other species and signals of repetitive DNA sequences were exclusively detected. It is a simple approach for identifying distribution of the repeated sequences on chromosomes.Comparative studies were performed by molecular banding among several species of the genera Tripsacum, Coix and Zea. The results showed most of the corresponding chromosomal regions among the tested species share hybridization signals of the repetitive sequences. When a given genomic DNA was hybridized to their own chromosomes, the signals were the strongest. Whileit was hybridized onto the chromosomes of the other tested species, the signals were much weaker. It showed variation of repetitive sequences was obvious in different species. In the molecular banding, hybridization signals were banding-like in shape on the whole and could be divided into interstitial, telomeric, centromeric bands and knobs. For centromeric and pericentromeric regions, the signals were much stronger in the hybridization with their own genomic DNA than in that with other genomic DNA, while for interstitial and telomeric regions, the signals showed no much more differences in strength for the different genomic DNA probes. The results showed repetitive sequences on centromeric and pericentromeric regions were less conservative than those on interstitial and terminal regions in the tested genera. The knob signals were very strong and clear only for the hybridization of Tripsacum dactyloides (2n=72) with genus Zea instead of Coix lacryma-jobi (2n=20) with them. This provides evidence that the genus Zea is closer to the genus Tripsacum than to the genus Coix evolutionally.2. Quantitative chromosome mapping is a new developed molecular cytogenetic technique in recent years. It is undertaken by quantitatively analyzing the intensely and faintly stained regions or bands on chromosomes using computer software. Accuracy of chromosomal identification could be notably improved by the quantitative analysis. Mitotic metaphase chromosomes of C. lacryma-jobi studied cytologically have been proved to be hardly distinguishable due to their morphological similarity and the lack of distinct chromosomal landmarks. In contrast to the metaphase chromosomes, prometaphase chromosomes of C. lacryma-jobi were longer and showed obvious differences among intense and faint regions stained by DAPI. The intensely and faintly stained regions corresponded to more condensed and more dispersed chromatin fibers along chromosomes, respectively. The uneven stained regions can be used as key markers to characterize chromosomes of C. lacryma-jobi and facilitate identification of homologous chromosomes. Distributional DAPI stained patterns along each chromosome were quantified by MetaMorph software. Based on the data besides chromosomal relative length and arm ratio, the quantitative chromosome map was constructed. The chromosome map could illustrate not only the more distinctive chromosome features, but also fractional length of differential condensation regions. The quantitative chromosome map could provide essential information...
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