Allan R. Lohe

What restricts the activity of mariner-like transposable elements?

A number of mechanisms have recently been described that might be important in restricting the level of activity of mariner-like transposable elements (MLEs) in natural populations. These mechanisms include overproduction inhibition, in which increasing the dose of transposase decreases net activity. Another mechanism is mediated by certain missense mutations, in which a mutant transposase protein impairs the activity of the wild-type transposase in heterozygous mutant/nonmutant genotypes. A further mechanism is the potential for transposase titration by defective elements that retain transposase binding activity. The issue of regulation is not only of theoretical importance in understanding the molecular and evolutionary genetics of MLEs, but also of practical significance in learning how best to use MLEs in the germline transformation of insect pests and disease vectors.

Return of the H-word (heterochromatin)

Recent advances in studies of yeast, Drosophila and humans have renewed interest in heterochromatin. These recent studies have demonstrated the interspersion and rapid spread of transposable elements into Drosophila heterochromatin; documented the requirement of heterochromatic genes for heterochromatin; identified heterochromatin-like regions in yeast chromosomes; confirmed an important role for satellite DNA in human centromere function; and suggested potential functions for heterochromatin-associated proteins.

Regulation of the transposable element mariner

The mariner/ Tc1 superfamily of transposable elements is widely distributed in animal genomes and is especially prevalent in insects. Their wide distribution results from their ability to be disseminated among hosts by horizontal transmission and also by their ability to persist in genomes through multiple speciation events. Although a great deal is known about the molecular mechanisms of transposition and excision, very little is known about the mechanisms by which transposition is controlled within genomes. The issue of mariner/Tc1 regulation is critical in view of the great interest in these elements as vectors for germline transformation of insect pests and vectors of human disease. Several potentially important regulatory mechanisms have been identified in studies of genetically engineered mariner elements. One mechanism is overproduction inhibition, in which excessive wild-type transposase reduces the rate of excision of a target element. A second mechanism is mediated by certain mutant transposase proteins, which antagonize the activity of the wild-type transposase. The latter process may help explain why the vast majority of MLEs in nature undergo ‘vertical inactivation’ by multiple mutations and, eventually, stochastic loss. Another potential mechanism of regulation may result from transposase titration by defective elements that retain their DNA binding sites and ability to transpose. There is also evidence that some mariner/Tc1 elements can be mobilized in a type of hybrid dysgenesis.

Towards a Drosophila genome map

A physical map of the genome of Drosophila melanogaster has been created using 965 yeast artificial chromosome (YAC) clones assigned to locations in the cytogenetic map by in situ hybridization with the polytene salivary gland chromosomes. Clones with insert sizes averaging about 200 kb, totaling 1.7 genome equivalents, have been mapped. More than 80% of the euchromatic genome is included in the mapped clones, and 75% of the euchromatic genome is included in 161 cytological contigs ranging in size up to 2.5 Mb (average size 510 kb). On the other hand, YAC coverage of the one-third of the genome constituting the heterochromatin is incomplete, and clones containing long tracts of highly repetitive simple satellite DNA sequences have not been recovered.

Mapping simple repeated DNA sequences in heterochromatin of Drosophila melanogaster: Genetics 134, 1149–1174

Heterochromatin in Drosophila has unusual genetic, cytological and molecular properties. Highly repeated DNA sequences (satellites) are the principal component of heterochromatin. Using probes from cloned satellites, we have constructed a chromosome map of 10 highly repeated, simple DNA sequences in heterochromatin of mitotic chromosomes of Drosophila melanogaster. Despite extensive sequence homology among some satellites, chromosomal locations could be distinguished by stringent in situ hybridizations for each satellite. Only two of the localizations previously determined using gradient-purified bulk satellite probes are correct. Eight new satellite localizations are presented, providing a megabase-level chromosome map of one-quarter of the genome. Five major satellites each exhibit a multi-chromosome distribution, and five minor satellites hybridize to single sites on the Y chromosome. Satellites closely related in sequence are often located near one another on the same chromosome. About 80% of Y chromosome DNA is composed of nine simple repeated sequences, in particular (AAGAC)n (8 Mb), (AAGAG)n (7 Mb) and (AATAT)n (6 Mb). Similarly, more than 70% of the DNA in chromosome 2 heterochromatin is composed of five simple repeated sequences. We have also generated a high resolution map of satellites in chromosome 2 heterochromatin, using a series of translocation chromosomes whose breakpoints in heterochromatin were ordered by N-banding. Finally, staining and banding patterns of heterochromatic regions are correlated with the locations of specific repeated DNA sequences. The basis for the cytochemical heterogeneity in banding appears to depend exclusively on the different satellite DNAs present in heterochromatin.

Genetic and epigenetic processes in seed development

Seed development has emerged as an important area of research in plant development. Recent research has highlighted the divergent reproductive strategies of the male and female genomes and interaction between genetic and epigenetic control mechanisms. Isolation of genes involved in embryo and endosperm development is leading to an understanding of the regulation of these processes at the molecular level. A thorough grasp of these processes will not only illuminate an important area of plant development but will also have an impact on agronomy by helping to facilitate food production. An understanding of seed development is also likely to clarify the molecular mechanisms of apomixis, a fascinating process of asexual seed production present in many plants.

Evolution of DNA in heterochromatin: the Drosophila melanogaster sibling species subgroup as a resource.

The Drosophila melanogasterspecies subgroup is a closely-knit collection of eight sibling species whose relationships are well defined. These species are too close for most evolutionary studies of euchromatic genes but are ideal to investigate the major changes that occur to DNA in heterochromatin over short periods during evolution. For example, it is not known whether the locations of genes in heterochromatin are conserved over this time. The 18S and 28S ribosomal RNA genes can be considered as genuine heterochromatic genes. In D. melanogasterthe rRNA genes are located at two sites, one each on the X and Y chromosome. In the other seven sibling species, rRNA genes are also located on the sex chromosomes but the positions often vary significantly, particularly on the Y. Furthermore, rDNA has been lost from the Y chromosome of both D. simulansand D. sechellia, presumably after separation of the line leading to present-day D. mauritiana.We conclude that changes to chromosomal position and copy number of rDNA arrays occur over much shorter evolutionary timespans than previously thought. In these respects the rDNA behaves more like the tandemly repeated satellite DNAs than euchromatic genes.

Demeter: On Seeds and Goddesses

The genetic and developmental processes that control grain and seed development have become a fertile area of research in recent years. Insights have been obtained on how two haploid genomes come together to form the diploid zygote and triploid endosperm inside the maternal organs that sustain them

Intertribal hybrid plants produced from crossing Arabidopsis thaliana with apomictic Boechera

Arabidopsis thaliana and Boechera belong to different tribes of the Brassicaceae and last shared a common ancestor 13–35 million years ago. A. thaliana reproduces sexually but some Boechera accessions reproduce by apomixis (asexual reproduction by seed). The two species are reproductively isolated, preventing introgression of the trait(s) controlling apomixis from Boechera into A. thaliana and their molecular characterisation. To identify if “escapers” from such hybridisation barriers exist, we crossed diploid or tetraploid A. thaliana mothers carrying a conditional male sterile mutation with a triploid Boechera apomict. These cross-pollinations generated zygotes and embryos. Most aborted or suffered multiple developmental defects at all stages of growth, but some seed matured and germinated. Seedlings grew slowly but eventually some developed into mature plants that were novel synthetic allopolyploid hybrids. With one exception, intertribal hybrids contained three Boechera plus either one or two A. thaliana genomes (depending on maternal ploidy) and were male and female sterile. The exception was a semi-fertile, sexual partial hybrid with one Boechera plus two A. thaliana genomes. The synthesis of “escapers” that survive rigorous early developmental challenges in crosses between A. thaliana and Boechera demonstrates that the inviability form of postzygotic reproductive isolation separating these distantly related species is not impenetrable. The recovery of a single semi-fertile partial hybrid also demonstrates that hybrid sterility, another form of postzygotic reproductive isolation, can be overcome between these species.