Homologue Pairing in Flies and Mammals: Gene Regulation When Two Are Involved

Chromosome pairing is usually discussed in the context of meiosis. Association of homologues in germ cells enables chromosome segregation and is necessary for fertility. A few organisms, such as flies, also pair their entire genomes in somatic cells. Most others, including mammals, display little homologue pairing outside of the germline. Experimental evidence from both flies and mammals suggests that communication between homologues contributes to normal genome regulation. This paper will contrast the role of pairing in transmitting information between homologues in flies and mammals. In mammals, somatic homologue pairing is tightly regulated, occurring at specific loci and in a developmentally regulated fashion. Inappropriate pairing, or loss of normal pairing, is associated with gene misregulation in some disease states. While homologue pairing in flies is capable of influencing gene expression, the significance of this for normal expression remains unknown. The sex chromosomes pose a particularly interesting situation, as females are able to pair X chromosomes, but males cannot. The contribution of homologue pairing to the biology of the X chromosome will also be discussed.

A Quantitative Measure of the Mitotic Pairing of Alleles in Drosophila melanogaster and the Influence of Structural Heterozygosity

In Drosophila there exist several examples of gene expression that can be modified by an interaction between alleles; this effect is known as transvection. The inference that alleles interact comes from the observations that homologous chromosomes pair in mitotically dividing cells, and that chromosome rearrangements can alter the phenotype produced by a pair of alleles. It is thought that heterozygous rearrangements impede the ability of alleles to pair and interact. However, because the existing data are inconsistent, this issue is not fully settled. By measuring the frequency of site-specific recombination between homologous chromosomes, we show that structural heterozygosity inhibits the pairing of alleles that lie distal to a rearrangement breakpoint. We suggest that some of the apparent conflicts may owe to variations in cell-cycle lengths in the tissues where the relevant allelic interactions occur. Cells with a longer cell cycle have more time to establish the normal pairing relationships that have been disturbed by rearrangements. In support, we show that Minute mutations, which slow the rate of cell division, partially restore a transvection effect that is disrupted by inversion heterozygosity.

Genetic evidence of Mutator-induced deletions in the short arm of chromosome 9 of maize. II. wd deletions.

Analyses of 113 putative Mutator-induced events involving the yg2 locus of chromosome 9 revealed that 11 of these events were deletions that produce albino seedlings when homozygous. This phenotype is characteristic of wd (white deficiency) deletions. All 11 wd-Mu deletions failed to complement wd1 and pyd1 (pale-yellow deficiency). Nine of the wd-Mu deletions were analyzed cytologically. Two were found to be terminal deletions and seven were internal deletions. Two of the seven had normal pairing throughout the terminal region involved in the pyd1 and wd1 deletions. Because genetic tests established that deletions were present in these two stocks, these deletions were probably too short to disrupt the pairing of the homologous chromosomes. Mechanisms by which the Mutator system might generate these deletions are discussed.

The influence of the Robertsonian translocation Rb(X.2)2Ad on anaphase I non-disjunction in male laboratory mice

SummaryA Robertsonian translocation in the mouse between theXchromosome and chromosome 2 is described. The male and female carriers of the Rb(X.2)2Ad were fertile. A homozygous/hemizygous line was maintained. The influence of theX-autosomal Robertsonian translocation on anaphase I non-disjunction in male mice was studied by chromosome counts in cells at metaphase II of meoisis and by assessment of aneuploid progeny. The results conclusively show that the inclusion of Rb2Ad in the male genome induces non-disjunction at the first meoitic division. In second metaphase cells the frequency of sex-chromosomal aneuploidy was 10·8%, and secondary spermatocytes containing two or no sex chromosome were equally frequent. The Rb2Ad males sired 3·9% sex-chromosome aneuploid progeny. The difference in aneuploidy frequencies in the germ cells and among the progeny suggests that the viability of XO and XXY individuals is reduced. The pairing configurations of chromosomes 2, Rb2Ad andYwere studied during meiotic prophase by light and electron microscopy. Trivalent pairing was seen in all well spread nuclei. Complete pairing of the acrocentric autosome 2 with the corresponding segment of the Rb2Ad chromosome was only seen in 3·2% of the cells analysed in the electron microscope. The pairing between theXand theYchromosome in the Rb2Ad males corresponded to that in males with normal karyotype. Reasons for sex-chromosomal non-disjunction despite the normal pairing pattern between the sex chromosomes may be seen in the terminal chiasma location coupled with the asynchronous separation of the sex chromosomes and the autosomes. The Rb2Ad chromosome can be useful for studies ofXinactivation, as a marker for parental derivation of theXchromosome and for mapping loci byin situhybridization.

Mathematical models for estimating preferential pairing and recombination in triploid hybrids

Different mathematical models for estimating pairing and recombination parameters for triploid hybrids with two-armed chromosomes are discussed. In most models all information on preferential pairing is contained in the ratio of trivalents and ring bivalents and can be estimated independently of the chiasmate association frequencies in the two chromosome arms. The single degree of freedom available only permits the estimation of ranges of the three possible pairing relations between the three genomes. Alternatively, a single parameter can be estimated when additional assumptions are made. When the ratio r between the frequencies of trivalents and ring bivalents, independent of arm length differences and with a theoretical maximum of 2, is about 1, the ranges of the frequencies of the three pairing combinations are wide. When r becomes smaller than 1, very soon the ranges become limited to values where one of the three is relatively large and positive and the other two negative and varying between equal to very different, depending on slight changes in the first. When the frequency of open bivalents is relatively high and the frequency of univalents low, there most probably is a difference in chiasmate association frequency between the two arms of the average chromosome and this difference can be quantified. When the number of univalents is only slightly higher than expected on the basis of the number of open bivalents, the reason may be that (quantifiably) more chiasmata are formed after bivalent pairing than after trivalent pairing within certain ranges of r, and certain ranges of the average chiasmate association frequency. When the excess of univalents is larger, this is best explained by a failure of entire chromosomes to find each other. This degree of pairing failure can be estimated. All models have been applied to the triploid hybrid between allotetraploid Trifolium repens and diploid T. nigrescens. Assuming that the two genomes of T. repens do not pair, which cannot be demonstrated with certainty, the two genomes pair with the nigrescens genome with frequencies of 0.828 and 0.171, respectively. Introgression then occurs into either genome but not with the same frequency. If the repens genomes pair, this would be caused by either genetic factors disturbing the normal pairing behaviour or the absence of strict homologues. Then the relative pairing frequency between T. nigrescens and one of the T. repens genomes would be 0.838 and on an average 0.081 between the two other combinations, with a possibly considerable but unknown difference. The high average chiasmate arm association frequency (0.650) suggests that affinity between the pairing genomes is not very low. The average two arms do not differ in chiasma frequency.Key words: triploid hybrids, preferential pairing, recombination, mathematical models, Trifolium.

Genetic variability in Muscari comosum L. (Liliaceae). II. Characterization and effects of the polymorphic variants of chromosome 2 on chiasmata formation

Muscari comosum L. (Liliaceae) displays a striking chromosomal polymorphism in the second largest chromosome. This polymorphism involves four cosmopolitan types. Two of these are shorter than the other two homologues. One of these is submetacentric (SSM) and the other is subtelocentric (SST). The two longer types also include a submetacentric (LSM) and a subtelocentric (LST) morph. Each of the two submetacentric chromosomes has one interstitial C-band in the short arm and each of the two subtelocentric morphs has an interstitial C-band in the long arm. The change of position of this interstitial C-band is most easily explained by a pericentric inversion. Furthermore, all four types of chromosome 2 have a centromeric C-band, while the two subtelocentrics have an additional terminal C-band in the long arm. The variability in the size of the second chromosome is most likely the consequence of an unequal interchange or an insertional translocation. The meiotic behaviour of the chromosome 2 bivalents in individuals heterozygous for the pericentric inversion is characterized by normal pairing between homologues with no inversion loops, though asynapsis was present in some meiocytes. Chiasmata are absent in two regions of chromosome 2 bivalents in these heterozygotes in which they regularly form in both classes of homozygotes. In individuals heterozygous for the long morphs of chromosome 2 the bivalents again showed normal pairing at pachytene, with chiasmata again absent in some regions in which they normally form. The net result is that homozygotes have significantly higher chiasmata frequencies than hterozygotes. Key words: genetic variability, chiasma formation, Muscari.

Evidence on the origin of the G genome in wheat: cytology and fertility of a T. timopheevi-like mutant

A spontaneous somatic mutant with Triticum timopheevi Zhuk. plant morphology was found in late tillers of a Triticum turgidum L. var. dicoccoides (Bowden) plant. Hybrids of the timopheevi-like mutant with T. turgidum L. var. dicocooides (Bowden), T. timopheevi Zhuk., and T. araraticum Jakubz. exhibited irregular pairing at meiosis and sterility, almost normal pairing and fertility, and relatively high pairing and sterility, respectively. This evidence plus that of karyotype changes in the somatic mutant indicated that it was differentiated from T. turgidum dicoccoides through chromosome interchanges. This mutation, believed to arise through chromosome rearrangement, is discussed in relation to the origin of the G genome of T. timopheevi.

Quantitative Analysis Of Meiotic Configurations In Tip Sterile Lines Of Hexaploid Wheat

In five tip sterile lines of hexaploid wheat (Kheri, Mexicani, Pavon-76, Seri-82 and Sonora-64) due to interchange of segments three types of meiotic configuration i.e. ring quadrivalent (oIV), chain quadrivalent (cIV) and trivalent plus univalent (III.I) were found at metaphase I. The observed and expected numbers of meiotic configurations in these translocated lines of wheat were found to following the equations based on normal pairing and non random distribution of chiasmata. Different configurations (oIV, cIV, III.I, oII, cII and I) were analyzed following Jackson and Murray’s (1986) formula. The results revealed that χ2-values for the translocated heterozygotes showed to be fit with the expectation. Data showed good agreement between numbers of expected and observed meiotic configuration and all the lines were in fit to the Ho with p > 0.05. DOI: http://dx.doi.org/10.3329/jles.v6i0.9715 JLES 2011 6: 13-18