Reduced DNA polytenization of a minichromosome region undergoing position-effect variegation in Drosophila

A position effect on the time of replication origin activation in yeast

The chromosomes of eukaryotes are characterized by the mosaic nature of their replication--large regions of DNA that replicate early in S phase are interspersed with regions that replicate late. This pattern of early and late synthesis appears to be the consequence of a temporal program that activates replication origins at different times. The basis of this temporal regulation in the yeast S. cerevisiae has been investigated by changing the chromosomal locations of two origins, one activated early in the S phase (ARS1) and one activated late (ARS501). We show that the cis-acting information controlling time of activation can be separated from the element that determines origin function. For the ARS501 origin, late activation appears to be a consequence of its proximity to the telomere.

There are two mechanisms of achiasmate segregation in Drosophila females, one of which requires heterochromatic homology

There are numerous examples of the regular segregation of achiasmate chromosomes at meiosis I in Drosophila melanogaster females. Classically, the choice of achiasmate segregational partners has been thought to be independent of homology, but rather made on the basis of availability or similarities in size and shape. To the contrary, we show here that heterochromatic homology plays a primary role in ensuring the proper segregation of achiasmate homologs. We observe that the heterochromatin of chromosome 4 functions as, or contains, a meiotic pairing site. We show that free duplications carrying the 4th chromosome pericentric heterochromatin induce high frequencies of 4th chromosome nondisjunction regardless of their size. Moreover, a duplication from which some of the 4th chromosome heterochromatin has been removed is unable to induce 4th chromosome nondisjunction. Similarly, in the absence of either euchromatic homology or a size similarity, duplications bearing the X chromosome heterochromatin also disrupt the segregation of two achiasmate X chromosome centromeres. Although heterochromatic regions are sufficient to conjoin nonexchange homologues, we confirm that the segregation of heterologous chromosomes is determined by size, shape, and availability. The meiotic mutation Axs differentiates between these two processes of achiasmate centromere coorientation by disrupting only the homology-dependent mechanism. Thus there are two different mechanisms by which achiasmate segregational partners are chosen. We propose that the absence of diplotene-diakinesis during female meiosis allows heterochromatic pairings to persist until prometaphase and thus to co-orient homologous centromeres. We also propose that heterologous disjunctions result from a separate and homology-independent process that likely occurs during prometaphase. The latter process, which may not require the physical association of segregational partners, is similar to those observed in many insects, in Saccharomyces cerevisiae and in C. elegans males. We also suggest that the physical basis of this process may reflect known properties of the Drosophila meiotic spindle.

Use of transgenic mice to infer the biological properties of small intestinal stem cells and to examine the lineage relationships of their descendants

Transgenes, composed of elements of the 5' nontranscribed region of the liver fatty acid-binding protein (L-FABP) gene linked to various reporters, have previously been used to explore the cellular, regional, and temporal differentiation of the mouse intestinal epithelium. In this report, we have analyzed a pedigree of L-FABP/human growth hormone (hGH) transgenic mice that display a stable, heritable, mosaic pattern of reporter expression: wholly hGH-positive or hGH-negative populations of differentiating enterocytes arise from hGH-positive or hGH-negative crypts, respectively, and migrate as vertical coherent bands up the villus producing striped (polyclonal) villi. The ability of enteroendocrine cells within a given villus stripe to support hGH expression coincides with the enterocytic reporter phenotype, suggesting that these two terminally differentiated cells arise from a common multipotent stem cell. hGH-negative crypts are nonrandomly distributed around each villus and their frequency increases along the duodenal-to-ileal axis. Statistical analysis of the observed villus striping pattern suggests that transgene expression is not independently determined in individual crypts but rather in multicrypt "patches." The intact endogenous mouse L-FABP gene (Fabpl) exhibits a similar striped villus pattern of expression in a portion of the distal small intestine. These studies indicate that Fabpl and L-FABP/hGH transgenes represent sensitive markers for exploring the biological properties of gut stem cells and how positional information is encoded in this rapidly and continuously renewing epithelium.

Relationship of eukaryotic DNA replication to committed gene expression: general theory for gene control

The historic arguments for the participation of eukaryotic DNA replication in the control of gene expression are reconsidered along with more recent evidence. An earlier view in which gene commitment was achieved with stable chromatin structures which required DNA replication to reset expression potential (D. D. Brown, Cell 37:359-365, 1984) is further considered. The participation of nonspecific stable repressor of gene activity (histones and other chromatin proteins), as previously proposed, is reexamined. The possible function of positive trans-acting factors is now further developed by considering evidence from DNA virus models. It is proposed that these positive factors act to control the initiation of replicon-specific DNA synthesis in the S phase (early or late replication timing). Stable chromatin assembles during replication into potentially active (early S) or inactive (late S) states with prevailing trans-acting factors (early) or repressing factors (late) and may asymmetrically commit daughter templates. This suggests logical schemes for programming differentiation based on replicons and trans-acting initiators. This proposal requires that DNA replication precede major changes in gene commitment. Prior evidence against a role for DNA replication during terminal differentiation is reexamined along with other results from terminal differentiation of lower eukaryotes. This leads to a proposal that DNA replication may yet underlie terminal gene commitment, but that for it to do so there must exist two distinct modes of replication control. In one mode (mitotic replication) replicon initiation is tightly linked to the cell cycle, whereas the other mode (terminal replication) initiation is not cell cycle restricted, is replicon specific, and can lead to a terminally differentiated state. Aberrant control of mitotic and terminal modes of DNA replication may underlie the transformed state. Implications of a replicon basis for chromatin structure-function and the evolution of metazoan organisms are considered.

Sex chromosomes and speciation

Studies of reproductive isolation between animal species have shown (i) that if one sex of the hybrids between two species is sterile or inviable, it is usually the heterogametic sex (Haldane's rule), and (ii) the genes on the sex chromosomes play a particularly large role in hybrid sterility and inviability. We propose an explanation for these two observations which is based on the changes in chromosome conformation which take place during gametogenesis. These changes are far greater in sex chromosomes than in autosomes. They are also greater in the heterogametic than in the homogametic sex. We suggest that the sensitivity of hybrids of the heterogametic sex to the genetic divergence that occurs during periods of population isolation is partly the result of the failure of their sex chromosomes to undergo appropriate conformational changes. This hypothesis explains why the sex chromosomes play a disproportionate role in post-zygotic, but not in pre-zygotic, isolation, and why often only the germ line is sensitive to hybridization.

A pairing-sensitive element that mediates trans-inactivation is associated with the Drosophila brown gene

Position-effect variegation in Drosophila is the mosaic expression of a gene juxtaposed to heterochromatin by chromosome rearrangement. The brown (bw+) gene is unusual in that variegating mutations are dominant, causing "trans-inactivation" of the homologous allele. We show that copies of bw+ transposed to ectopic sites are not trans-inactivated by rearrangements affecting the endogenous gene. However, when position-effect variegation is induced on an ectopic copy by chromosome rearrangement, the allele on its paired homolog is trans-inactivated, whereas other copies of bw+ are not. This confirms that trans-inactivation is "chromosome local" and maps the responsive element to the immediate vicinity of brown. Subsequent P-transposase-induced deletions within the ectopic copy in cis to the rearrangement breakpoint caused partial suppression of trans-inactivation. Surprisingly, the amount of suppression was correlated with deletion size, with some degree of trans-inactivation persisting even when the P[bw+] transposon was completely excised. The chromosome-local nature of the phenomenon and its extreme sensitivity to small disruptions of somatic pairing leads to a model in which a regulator of the brown gene is inactivated by direct contact with heterochromatic proteins.

Position-effect variegation after 60 years

Position-effect variegation in Drosophila--the mosaic expression of a gene placed adjacent to a junction between euchromatin and heterochromatin--remains an enigma. However, new insights are being gained from recent studies of genetic modifiers, new model systems, and variegating genes showing exceptional behavior.