Controversies over the molecular clock hypothesis were reviewed. Since it is evident that the molecular clock does not hold in an exact sense, accounting for evolution of the rate of molecular evolution is a prerequisite when estimating divergence times with molecular sequences. Recently proposed statistical methods that account for this rate variation are overviewed and one of these procedures is applied to the mitochondrial protein sequences and to the nuclear gene sequences from many mammalian species in order to estimate the time scale of eutherian evolution. This Bayesian method not only takes account of the variation of molecular evolutionary rate among lineages and among genes, but it also incorporates fossil evidence via constraints on node times. With denser taxonomic sampling and a more realistic model of molecular evolution, this Bayesian approach is expected to increase the accuracy of divergence time estimates.

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A new esterase isozyme locus, Est10, with 6 alleles including the null form, has been found in rice by using polyacrylamide gel electrophoresis. Thirty F₂ populations of all possible combinations between 5 different band morphs were studied. The segregation pattern indicated that bands 1, 2, 3, 4, and the null form (0) were allelic with each other. The alleles of Est10 were distributed at different frequencies among different varietal groups of rice and also between cultivated rice and its wild relatives (Oryza rufipogon Griff.). Alleles 1 and 2 were frequently found in Japonica and Indica types, respectively. Allele 3 showed a high frequency in Aus and Boro, both Indica types cultivated in South Asia. Allele 4 was frequent in wild rice O. rufipogon. Judging from the linkage between Est10 and RFLP marker RG220 and isozyme marker Est5, Est10 is located on chromosome 1. The importance of this locus in evolutionary studies of rice is discussed.

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The genome of common wheat has evolved through allopolyploidization of three ancestral diploid genomes. A previously identified restriction fragment length polymorphism (RFLP) marker, pTag546, has the unique feature of showing hypervariability among closely related common wheat cultivars. To understand the origin and the mode of dispersal of this hypervariable sequence in the wheat genome, the distribution and structure of the homologous sequences were studied using ancestral diploid species, tetraploid disomic substitution lines and synthetic hexaploid lines. Comparative Southern blot and PCR analyses suggested that pTag546 homologs in the tetraploid and hexaploid wheat were derived from the S genome of Aegilops speltoides. Some pTag546 homologs were found to have transposed to A and D genomes in polyploid wheat. Evidence of transposition and elimination in some synthetic hexaploid lines was also obtained by comparing their copy numbers with those in the parental lines. Southern blot analysis of a genomic clone using a contiguous subset of sequences as probes revealed a core region of hypervariability that coincided with the region containing pTag546. No obvious structural characteristics that could explain the hypervariability, however, were found around the pTag546 sequence, except for accumulation of small repetitive sequences at one border. It was concluded that pTag546 increased its copy number through yet unknown mechanism(s) of transposition to various chromosomal locations over the period of allopolyploid evolution and during the artificial genome manipulation in wheat.

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To understand the genomic organization of Diospyros species with different ploidy levels, we cloned three different repetitive DNAs and compared their genomic distributions in ten Diospyros species, including hexaploid D. kaki and D. virginiana. Genomic Southern hybridization demonstrated that the EcoRV-repetitive DNA was present in tandem in the genomes of D. glandulosa (2n=2x=30), D. oleifera (2n=2x=30), D. lotus (2n=2x=30), D. virginiana (2n=6x=90) and D. kaki (2n=6x=90). All of these species except D. virginiana also contained the HincII-repetitive DNA in tandem. Fluorescent in situ hybridization showed that the EcoRV- and HincII-repetitive DNAs were predominantly located at the proximal or centromeric regions of chromosomes. The DraI-repetitive sequence cloned from D. ehretioides (2n=2x=30) was not found in the other Diospyros species tested. This suggests that D. ehretioides has a genomic organization different from that of the other Diospyros species. Speciation of hexaploid Diospyros species is also discussed with respect to the genomic distribution of the three repetitive DNAs cloned.

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The molecular evolution of the histone multigene family was studied by cloning and determining the nucleotide sequences of the histone 3 genes in seven Drosophila species, D. takahashii, D. lutescens, D. ficusphila, D. persimilis, D.pseudoobscura, D. americana and D. immigrans. CT repeats, a TATA box and an AGTG motif in the 5' region, and a hairpin loop and purine-rich motifs (CAA(T/G)GAGA) in the 3' region were conserved even in distantly related species. In D. hydei and D.americana, the GC content at the third codon position in the protein coding region was relatively low (49% and 45%), while in D. takahashii and D. lutescens it was relatively high (64% and 65%). The non- significant correlation between the GC contents in the 3' region and at the third codon position as well as the evidence of less constraint in the 3' region suggested that mutational bias may not be the major mechanism responsible for the biased nucleotide change at the third codon position or for codon usage bias.

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