Orientation and segregation of a micromanipulated multivalent: familiar principles, divergent outcomes

Anaphase A

Anaphase A is the motion of recently separated chromosomes to the spindle pole they face. It is accompanied by the shortening of kinetochore-attached microtubules. The requisite tubulin depolymerization may occur at kinetochores, at poles, or both, depending on the species and/or the time in mitosis. These depolymerization events are local and suggest that cells regulate microtubule dynamics in specific places, presumably by the localization of relevant enzymes and microtubule-associated proteins to specific loci, such as pericentriolar material and outer kinetochores. Motor enzymes can contribute to anaphase A, both by altering microtubule stability and by pushing or pulling microtubules through the cell. The generation of force on chromosomes requires couplings that can both withstand the considerable force that spindles can generate and simultaneously permit tubulin addition and loss. This chapter reviews literature on the molecules that regulate anaphase microtubule dynamics, couple dynamic microtubules to kinetochores and poles, and generate forces for microtubule and chromosome motion.

Unraveling the Hexaploid Sweetpotato Inheritance Using Ultra-Dense Multilocus Mapping

The hexaploid sweetpotato (Ipomoea batatas (L.) Lam., 2n = 6x = 90) is an important staple food crop worldwide and plays a vital role in alleviating famine in developing countries. Due to its high ploidy level, genetic studies in sweetpotato lag behind major diploid crops significantly. We built an ultra-dense multilocus integrated genetic map and characterized the inheritance system in a sweetpotato full-sib family using our newly developed software, MAPpoly. The resulting genetic map revealed 96.5% collinearity between I. batatas and its diploid relative I. trifida We computed the genotypic probabilities across the whole genome for all individuals in the mapping population and inferred their complete hexaploid haplotypes. We provide evidence that most of the meiotic configurations (73.3%) were resolved in bivalents, although a small portion of multivalent signatures (15.7%), among other inconclusive configurations (11.0%), were also observed. Except for low levels of preferential pairing in linkage group 2, we observed a hexasomic inheritance mechanism in all linkage groups. We propose that the hexasomic-bivalent inheritance promotes stability to the allelic transmission in sweetpotato.

Unraveling the Hexaploid Sweetpotato Inheritance Using Ultra-Dense Multilocus Mapping

The hexaploid sweetpotato (Ipomoea batatas (L.) Lam., 2n = 6x = 90) is an important staple food crop worldwide and plays a vital role in alleviating famine in developing countries. Due to its high ploidy level, genetic studies in sweetpotato lag behind major diploid crops significantly. We built an ultra-dense multilocus integrated genetic map and characterized the inheritance system in a sweetpotato full-sib family using our newly developed software, MAPpoly. The resulting genetic map revealed 96.5% collinearity between I. batatas and its diploid relative I. trifida We computed the genotypic probabilities across the whole genome for all individuals in the mapping population and inferred their complete hexaploid haplotypes. We provide evidence that most of the meiotic configurations (73.3%) were resolved in bivalents, although a small portion of multivalent signatures (15.7%), among other inconclusive configurations (11.0%), were also observed. Except for low levels of preferential pairing in linkage group 2, we observed a hexasomic inheritance mechanism in all linkage groups. We propose that the hexasomic-bivalent inheritance promotes stability to the allelic transmission in sweetpotato.

Modeling of chromosome motility during mitosis

Chromosome motility is a highly regulated and complex process that ultimately achieves proper segregation of the replicated genome. Recent modeling studies provide a computational framework for investigating how microtubule assembly dynamics, motor protein activity and mitotic spindle mechanical properties are integrated to drive chromosome motility. Among other things, these studies show that metaphase chromosome oscillations can be explained by a range of assumptions, and that non-oscillatory states can be achieved with modest changes to the model parameters. In addition, recent microscopy studies provide new insight into the nature of the coupling between force on the kinetochore and kinetochore-microtubule assembly/disassembly. Together, these studies facilitate advancement toward a unified model that quantitatively predicts chromosome motility.

Chromosomal strategies for adaptation to univalency

The orientation and segregation behaviour of different types of univalents, namely sex chromosomes, B chromosomes and autosomal univalents, were analysed in living spermatocytes of eight evolutionarily distant grasshopper species. The meiotic behaviour of each univalent was characterized in terms of velocity of prometaphase movements, frequency of reorientations, types of final orientation at metaphase I and modes of segregation at anaphase I. All these features were found to vary between different univalents. Certain combinations of these traits, defining a 'chromosomal strategy', appear commonly together in certain chromosome types, indicating that they are the result of selection acting on the chromosomes to increase transmission effectiveness. The sex univalents show in general a strategy in which all the features favouring an eventual equational segregation at anaphase I tend to be minimized. There is much more variation in behaviour among B chromosomes than among X chromosomes, which is a reflection of their heterogeneous nature. Induced autosomal univalents are studied in Locusta migratoria. They show a very irregular behaviour, indicating their lack of adaptation to univalency.

A comparative study of orientation at behavior of univalent in living grasshopper spermatocytes

Orientational movements and modes of segregation at anaphase I were analyzed in three different types of univalents in living spermatocytes of the grasshopper species Eyprepocnemis plorans, namely the sex univalent, three types of accessory chromosomes and spontaneous and induced autosomal univalents. When two or more univalents were present in the same spindle, their dynamics were directly compared. Chromosomes may show variable velocity and number of reorientations: the X and the most common B types (B1 and B2) are slow and rarely reorient, a more geographically restricted B (B5) is faster and reorients more often, and autosomal univalents are the fastest and show the highest frequency of reorientations. Nonetheless, the X and the accessories are rigorously reductional at anaphase I whereas autosomal univalents often fail to migrate or divide equationally. This indicates that orientational and segregational behavior are controlled mainly by chromosomal rather than cellular characteristics and that chromosomes may display a great variety of strategies to achieve regular segregation.

The spatial arrangement of Allium triquetrum chain quadrivalents at metaphase I as reviewed by confocal microscopy

We examined the three-dimensional arrangement of bivalents and, in particular, a chain of four chromosomes (chain quadrivalent) in the metaphase I spindle of pollen mother cells of Allium triquetrum by confocal microscopy. Firstly, we show by optical sectioning and three-dimensional image reconstruction that the cooriented pairs of centromeres of all seven bivalents lie virtually parallel to each other in the metaphase I spindle, parallel to the long axis of the spindle. Secondly, we likewise show that the four centromeres of the chain quadrivalent are aligned in the metaphase I spindle in, essentially, a two-dimensional array, not in a three-dimensional array, as proposed by some other authors. This two-dimensionality has its basis, we argue, in the principle that poleward directed spindle forces minimise centromere-to-pole distances and therefore align pairs of centromeres connected to opposite poles most axially (vertically) in the spindle. These distances are minimised for the quadrivalent as a whole only when it lies in two dimensions, i.e. in a plane parallel to the spindle axis.

Meiotic analysis by FISH of a human male 46,XY,t(15;20)(q11.2;q11.2) translocation heterozygote: quadrivalent configuration, orientation and first meiotic segregation

Understanding the segregational behaviour of reciprocal translocations in man is of both theoretical and clinical importance. Generally, information for genetic counselling is obtained from empirical data although knowledge of gametic output can now be obtained by karyotyping individual human spermatozoa. However, neither empirical studies nor sperm karyotyping data provide detailed information on how the combinations of normal, balanced and unbalanced gametes arise. For this knowledge of quadrivalent orientation and first meiotic segregation is required. We have used dual colour fluorescence in situ hybridisation (FISH) to identify normal and derived chromosomes during meiosis in testicular biopsy material from a 46,XY,t(15;20)(q11.2;q11.2) heterozygote. We were able to determine the frequencies of different quadrivalent structures at first metaphase (MI) and the proportion of first meiotic divisions subject to interstitial chiasmata. Having identified all 2:2, 3:1 and 4:0 segregation products at second metaphase, it was possible to correlate segregation categories with the various forms of MI quadrivalent possibly indicating their modes of orientation. Finally the ratios of normal:balanced:unbalanced gametes expected to be produced by this translocation heterozygote were calculated.