Over the last 10,000 years, crop domestication has been the single most important human cultural development. Grasses are prominent among these crops, and provide the vast majority of the world's food. Similar traits have been selected during the domestication and breeding of these critically important grasses, and since they share a similar complement of genes, the same set of genes may have been selected. Even though the process of domestication occurred over the same 5000 to 10,000 year period, the domesticated grasses have major differences in genome structure, diversity, and life history. Molecular investigations of grass domestication have succeeded in identifying progenitor species and are beginning to catalog genetic resources. Additionally, research is now elucidating some of the basic processes by which crops have evolved over the last few millennia. In this review, we discuss our present knowledge of molecular diversity among the grass crops and relate that diversity to the genes involved in domestication and to yield gains. Understanding the connection between diversity and genome structure will be critical to future crop breeding.

The extent of imprinting at R-r, frequency of paramutation at B-Intense and Pl, and epigenetic silencing of Mu transposons were evaluated in the W23 and A188 inbred lines of maize. All types of epigenetic phenomena affecting these loci of the anthocyanin pathway occurred more frequently in the W23 inbred line. Absence of down-regulation was dominant in F1 hybrid progeny. Identical alleles programme lower anthocyanin accumulation in A188 than in W23, and A188 plants develop more rapidly than W23. The possibilities that specific genetic factors, intrinsic gene expression levels and/or the rapidity of the life cycle modulate epigenetic gene controls are discussed.

Chromosomal distribution of transposable elements (TEs) Osvaldo and blanco in D. buzzatii was studied in three original natural populations from Argentina (Berna, Puerto Tirol and La Nostalgia) and a colonizer population from the Iberian Peninsula (Carboneras). The Spanish population showed significant differences for Osvaldo and blanco copy numbers when we compared the X chromosome and the autosomes; but it is mainly the accumulation of copies in chromosome 2, where most sites with high insertion frequency were located, that causes the discrepancy with the negative selection model. We found no significant differences in TE frequency between chromosomal regions with different exchange rates, and no evident accumulation of TE was detected within chromosomal inversions where recombination rate is reduced. The Carboneras population shows euchromatic sites of Osvaldo and blanco with high occupancy and others with low copy number. On the contrary, Argentinian populations show only a generalized low occupancy per insertion site. Moreover, the mean copy number of both elements is higher in Spain than in Argentina. All these results suggest an important role of the colonization process in the distribution of TEs. The increase in the copy number of the TEs analysed and their elevated frequency in some chromosomal sites in Carboneras is, most probably, a sequel of the founder event and drift that took place at the time of the colonization of the Old World by D. buzzatii from the New World some 300 years ago.

In Drosophila melanogaster, the PGal4 transposon inserted at the chromosomal site 86E1-2 is associated with the Voila1 allele that causes multiple phenotypes. Homozygous Voila1/1 flies rarely reach adulthood and heterozygous Voila1/+ adult males display strong homosexual courtship behaviour. Both normal behavioural and developmental phenotypes were rescued by remobilizing the PGal4 element. Yet, the rescue of heterosexual courtship and of adult viability did not occur in the same strains, indicating that these defects have different genetic origins. Furthermore, many strains showed a partial rescue of both characters. Molecular analysis revealed that the PGal4 transposon is inserted upstream of the 5′UTR of the prospero gene. The excision strains with no detectable fragment of the PGal4 transposon remaining showed a rescued viability for homozygote adults. Moreover, the developmental period with the highest homozygote lethality was correlated with the size of PGal4 element that remained inserted at the Voila locus. This suggests a relationship between developmental viability and the amount of DNA inserted within the promoter of prospero.

Fifteen polymorphic microsatellite markers were used to establish linkage groups and relative rates of recombination in male and female Myzus persicae (Sulzer) (Hemiptera: Aphididae) (peach-potato aphid). We cloned nine markers from M. persicae and for these we report primer sequences and levels of allelic diversity and heterozygosity in four Australian M. persicae populations. Of the remaining six loci, four loci, also cloned from M. persicae, were obtained from G. Malarky (Natural History Museum, London) and two loci from Sitobion miscanthi were used. Additionally, the primer sequences of locus M77, a locus monomorphic in M. persicae but polymorphic in the closely related Myzus antirrhinii, are presented. Eleven of the 15 polymorphic markers were autosomal and four were X-linked. A linkage analysis was performed on a European pedigree of aphids containing five families with between seven and 11 offspring each. There was no linkage between any loci in females. In males, several pairwise comparisons yielded no recombinant offspring. With the exception of locus M40, these observations were supported in a linkage analysis performed on larger families produced from Australian M. persicae crosses. Locus M40 showed segregation consistent with involvement in a translocation between autosomes 1 and 3 in European samples but not in the Australian samples. From the Australian crosses we report an absence of recombination in males but high recombination rates in females. One X chromosome and four autosomal linkage groups were identified and tentatively assigned to chromosomes. The relevance of achiasmate meiosis to the evolution of sex is discussed.

Theoretical population genetic studies of transposable elements focus almost exclusively on random mating species, whereas many plants reproduce through partial or substantial self-fertilization. Here I develop computer simulation and analytic approximations of simplified element dynamics (transposition balanced by selective elimination) in partially self-fertilizing populations, using Ty1-copia elements for biological inspiration. Under the most plausible models and parameter values, element numbers decrease with self-fertilization when element insertions are deleterious, but may increase when ectopic exchange regulates element number. Conclusions for models of ectopic exchange depend in part on parameters for which little firm empirical evidence is available. Small changes in selfing rate can lead to abrupt changes in element number when homozygous and heterozygous elements have markedly different fitness effects. Equilibrium element numbers can be sensitive to population size, especially at high selfing rates. Elements are frequently lost in small highly selfing populations under the deleterious insertion model. In contrast, small highly selfing populations can accumulate very large numbers of elements under ectopic exchange. Empirical data on element number and localization in plants with different mating systems suggests that deleterious insertion, rather than ectopic exchange, may regulate element number. Limitations to available empirical data, especially the lack of comparison between closely related species differing in mating system, mean that this conclusion is tentative.

The effects of reproductive compensation on the population genetics of sex-linked recessive lethal mutations are investigated. Simple equations are presented which describe these effects, and so complement existing population genetic theory. More importantly, this type of mutation is responsible for several severe human genetic diseases such as Duchenne muscular dystrophy. It is argued that the applications of three modern reproductive technologies – effective family planning, in utero diagnosis with termination, and embryo sexing – will lead to reproductive compensation. The adoption of any of these technologies may rapidly elevate the frequencies of those mutations which are lethal in childhood. This increase is large, in the order of 33% upwards, and occurs rapidly over two to five generations. It also depends on the source of mutations, the effect being larger if most mutations are paternal. In utero diagnosis and/or embryo sexing increase the frequency of the mutation, but simultaneously decrease disease incidence by preventing the birth of affected offspring. In contrast, effective family planning may rapidly increase both mutation frequency and disease incidence.

We propose a method that minimizes the rate of inbreeding (ΔF) for small unselected populations with overlapping generations and several reproductive age classes. It minimizes the increase in coancestry of parents and optimizes the contribution of each selection candidate. The carrying capacity of the population is limited to a fixed number of animals per year. When survival rate equalled 100%, only animals from the oldest age class were selected, which maximized the number of parents per generation, slowed down the turnover of generations and minimized the increase of coancestry across sublines. However, the population became split into sublines separated by age classes, which substantially increased inbreeding within sublines. Sublines were prevented by a restriction of selecting at least one sire and one dam from the second-oldest age class, which resulted in an L times lower ΔF, where L equals the average generation interval of sires and dams. Minimum coancestry mating resulted in lower levels of inbreeding than random mating, but ΔF was approximately the same. For schemes where the oldest animals were selected, ΔF increased by 18–52% compared with the proposed method.