The genome sizes of a Begonia collection comprising 37 species and 23 hybrids of African, Asiatic, Middle American, and South American origin were screened using flow cytometry. Within the collection, 1C values varied between 0.23 and 1.46 pg DNA. Genome sizes were, in most cases, not positively correlated with chromosome number, but with pollen size. A 12-fold difference in mean chromosome size was found between the genotypes with the largest and smallest chromosomes. In general, chromosomes from South American genotypes were smaller than chromosomes of African, Asian, or Middle American genotypes, except for B. boliviensis and B. pearcei. Cytological chromosome studies in different genotypes showed variable chromosome numbers, length, width, and total chromosome volume, which confirmed the diversity in genome size. Large secondary constrictions were present in several investigated genotypes. These data show that chromosome number and structure exhibit a great deal of variation within the genus Begonia, and likely help to explain the large number of taxa found within the genus.

The pennate diatom Seminavis robusta, characterized by an archetypical diatom life cycle including a heterothallic mating system, is emerging as a model system for studying the molecular regulation of the diatom cell and life cycle. One of its main advantages compared with other diatom model systems is that sexual crosses can be made routinely, offering unprecedented possibilities for forward genetics. To date, nothing is known about the genetic basis of sex determination in diatoms. Here, we report on the construction of mating type-specific linkage maps for S. robusta, and use them to identify a single locus sex determination system in this diatom. We identified 13 mating type plus and 15 mating type minus linkage groups obtained from the analysis of 463 AFLP markers segregating in a full-sib family, covering 963.7 and 972.2 cM, respectively. Five linkage group pairs could be identified as putative homologues. The mating type phenotype mapped as a monogenic trait, disclosing the mating type plus as the heterogametic sex. This study provides the first evidence for a genetic sex determining mechanism in a diatom.

Tetraploids and mixoploids were induced in several Spathiphyllum wallisii genotypes through in vitro application of mitosis inhibitors. Flow cytometry of leaves enabled the identification of sectorial hybrids, whereas microscopic nuclei research combined with root flow cytometry was required to provide insights in the histogenic composition of the mixoploids and to identify periclinal chimeras. Microscopic observation of epidermal or parenchymatic cell areas or the average cell thickness did not allow unequivocal characterization. However, direct visualization of diploid and tetraploid cells was enabled by analysis of nuclear areas. Root analysis was performed by classical fluorescence of metaphase chromosome spreads. This study showed the necessity of microscopic techniques for a thorough characterization of mixoploids. It also demonstrated the potential of the combination of these techniques with flow cytometry to unravel the exact effect of mitosis inhibitors on monocot plants. Finally, it might help to optimize the tetraploidization efficiency.