Syncyte formation in the microsporangium of Chrysanthemum (Asteraceae): a pathway to infraspecific polyploidy

Polyploidy, which is thought to have played an important role in plant evolution and speciation, is prevalent in Chrysanthemum (x = 9). In fact, polyploid series are known in C. zawadskii (2x, 4x, 6x, 8x, and 10x) and C. indicum (2x, 4x, and 6x), but the mechanism by which polyploidization occurs is unknown. Here we show that in diploid individuals of both C. zawadskii and C. indicum, the fusion between two adjacent pollen mother cells (PMCs) occurs at a frequency of 1.1-1.3% early in the first meiotic division. While possessing the chromosomes of both PMCs, the fused cell or syncyte undertakes subsequent meiotic division processes as a single large PMC, producing four 2n pollen grains that are able to germinate. Despite their low frequency, syncyte formation may have played a major role in the production of infraspecific polyploids in Chrysanthemum.

Cytogenetic variability in several oil seed rape cultivars

Meiotic study was performed in 23 oil seed rape (Brassica napus) cultivars considering chiasma frequency and distribution as well as chromosomes pairing. All cultivars possessed n = 19 chromosome number (4x). Chromosome stickiness occurred in most of the cultivars from early prophase to late telophase-II leading to the formation of laggard chromosomes and micronuclei. The cultivars studied differed significantly in chiasma frequency and distribution as well as chromosomes association indicating their genomic differences. Cluster analysis and ordination based on principal components analysis grouped those cultivars showing meiotic similarities. Some of the cultivars showed the occurrence of B-chromosomes which significantly changed chiasma frequency and chromosome pairing in the cells possessing them. Unreduced meiocytes and pollen grains were observed in some of the cultivars due to cytomixis, anaphase failure and desynapsis.

Hypotheses for the functions of intercellular bridges in male germ cell development and its cellular mechanisms

In oogamous reproduction of multicellular organisms, a striking phenomenon is the prevailing synchronous development of male germ cells connected by wide intercellular bridges (IBs, 0.1-2 microm), which is well conserved in both animal and plant species ranging from algae to human. In the literature, IBs are believed either to allow genetically segregated haploid spermatids to share diploid gene products after meiosis, or to mediate rapid transfer of some vital signals or nutrients. Although intercellular sharing of gene transcripts has experimental evidences, these hypotheses are still not satisfactory. To explore the unknown roles of IB, we assume that developing male germ cells may be especially sensitive to stochastic gene expression to become heterogeneous. To achieve best gamete quality, such heterogeneity must be eliminated so that relatively uniform gametes with normal functions can be produced. Development within a common syncytium may be the only way for this purpose. The process may require not only the intercellular exchange of a few molecular signals but also the mixing of protoplasm between the connected cells so that they have similar levels/states of mRNAs, proteins and organelles, which can be achieved only through wide IBs. This hypothesis can explain some quite intriguing aspects of male gametogenesis and provide unique predictions that can be tested experimentally.