The complete process of meiosis was investigated in Pinus wallichiana, P. strobus and their artificial hybrid （F1） using microsporocytes. It is revealed that there were slightly lower chiasma frequency, lower ring bivalent frequency, lower meiotic index and distinctly higher frequency of aberrance （chromosomal bridges, fragments or micronuclei） in pollen mother cells （PMCs） of the hybrid （F1） than those of the parental species, which showed a certain degree of differentiation between homologous chromosomes of the two parents. However, relatively higher frequency of ring bivalents and higher meiotic index in all the three entities indicate the great stability of genomes of parental species, and the differentiation of genomes between the two parents must have been slight. Total nineteen signal loci of 18S rDNA were observed in nine bivalents of the hybrid （F1）, among which one bivalent bears two loci, while the others have only one. It is suggested that distinct differentiation at genetic level existed in homologous chromosomes of the two parental species, whereas only slight differentiation at karyotypic and genomic levels take place between the parent species.
Hui-Sheng DengDa-Ming ZhangCheng-Xin FuDe-Yuan Hong
The paper introduces the classical ABC model of floral development and thereafter ABCD, ABCDE and quartet models, and presents achievements in the studies on floral evolution such as the improved understanding on the relationship of reproductive organs between gnetophytes and angiosperms, new results in perianth evolution and identified homology of floral organs between dicots and monocots. The evo-devo studies on plant taxa at different evolutionary levels are useful to better understanding the homology of floral organs, and to clarifying the mysteries of the origin and sub-sequent diversification of flowers.
LI Guisheng, MENG Zheng, KONG Hongzhi, CHEN Zhiduan & LU Anming Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China Correspondence should be addressed to Meng Zheng (e-mail: zhmeng@ ns.ibcas.ac.cn) and Lu Anming (e-mail:anmin@ns.ibcas.ac.cn)
The process of flowering is controlled by a hierarchy of floral genes that act as flowering time genes, inflorescence/floral meristem Identity genes, and/or floral organ-identity genes. The most important and well-characterized floral genes are those that belong to the MADS-box family of transcription factors. Compelling evidence suggests that floral MADS-box genes have experienced a few large-scale duplication events. In particular, the precore eudicot duplication events have been considered to correlate with the emergence and diversification of core eudicots. Duplication of floral MADS-box genes has also been documented in monocots, particularly In grasses, although a systematic study is lacking. In the present study, by conducting extensive phylogenetlc analyses, we identified pre-Poaceae gene duplication events in each of the AP1, P1, AG, AGL11, AGL2/3/4, and AGL9gene lineages. Comparative genomic studies further indicated that some of these duplications actually resulted from the genome doubling event that occurred 66-70 million years ago （MYA）. In addition, we found that after gene duplication, exonization （of intron sequences） and pseudoexonization （of exon sequences） have contributed to the divergence of duplicate genes in sequence structure and, possibly, gene function.
The meiotic behavior of 10 taxa （nine species and one variety） of the genus Pinus was investigated using pollen mother cells （PMCs） to reveal the differentiation among karyotypes. Chromosome spreads were prepared by conventional squashing. The meiotic index and the average configuration were higher, whereas the frequency of aberrance （chromosomal bridges, fragments, or micronuclei） was lower, in all 10 taxa compared with other gynmosperms. The meiotic index, average configuration, and frequency of irregularity were found to be uniform among the species. It was shown that the genomes of the Pinus species investigated were highly stable, confirming results of previous mitotic analyses in this genus. However, slight differentiation of homologous chromosomes among genomes was revealed by analysis of meiotic configurations in Pinus nigra var. poiretiana. Quadrivalents were observed in 9.31% of PMCs in this species. This is the first time that quadrivalents have been observed in gymnosperms.
Hui-Sheng DENGDa-Ming ZHANGDe-Yuan HONGCheng-Xin FU
Gene duplication plays an important role in the evolution of organisms by allowing functional innovation and the divergence of duplicate genes. Previous studies found two PI-like genes in grass species, sug- gesting functional divergence between the paralogous copies. Here, we reconstructed the evolutionary history of two PI genes from major lineages of grasses and other monocot species, and demonstrated that two PI genes （PI1 and PI2） arose from a whole genome duplication that occurred in a common ancestor of extant grasses. Molecular evolutionary analyses at the family and tribal levels found strong purifying selection acting on two genes in grasses, consistent with the conserved class B function of the PI genes. Importantly, we detected different patterns of selective relaxation between the duplicated PI genes although no signature of positive selection was found. Likelihood ratio tests revealed that the ω ratio for M domain is significantly higher in PI1 than in PI2 but that for K domain is significantly higher in PI2 than in PI1. These findings imply that complementary selective relaxation occurs in two PI genes after duplication, and provide additional molecular evidence for the subfunctionalization of the duplicated PI genes in grasses.
Cereal genes are classified into two distinct classes according to the guanine-cytosine （GC） content at the third codon sites （GC3）. Natural selection and mutation bias have been proposed to affect the GC content. However, there has been controversy about the cause of GC variation. Here, we characterized the GC content of 1 092 paralogs and other single-copy genes in the duplicated chromosomal regions of the rice genome （ssp. indica） and classified the paralogs into GC3-rich and GC3-poor groups. By referring to out-group sequences from Arabidopsis and maize, we confirmed that the average synonymous substitution rate of the GC3-rich genes is significantly lower than that of the GC3-poor genes. Furthermore, we explored the other possible factors corresponding to the GC variation including the length of coding sequences, the number of exons in each gene, the number of genes in each family, the location of genes on chromosomes and the protein functions. Consequently, we propose that natural selection rather than mutation bias was the primary cause of the GC variation.
Xiaoli ShiXiyin WangZhe LiQihui ZhuJi YangSong GeJingchu Luo
