B Chromosomes in Free-Living Flatworms of the Genus Macrostomum (Platyhelminthes, Macrostomorpha)

B chromosomes (Bs) or supernumerary chromosomes are extra chromosomes in the species karyotype that can vary in its copy number. Bs are widespread in eukaryotes. Usually, the Bs of specimens collected from natural populations are the object of the B chromosome studies. We applied another approach analyzing the Bs in animals maintained under the laboratory conditions as lines and cultures. In this study, three species of the Macrostomum genus that underwent a recent whole-genome duplication (WGD) were involved. In laboratory lines of M. lignano and M. janickei, the frequency of Bs was less than 1%, while in the laboratory culture of M. mirumnovem, it was nearer 30%. Their number in specimens of the culture varied from 1 to 14. Mosaicism on Bs was discovered in parts of these animals. We analyzed the distribution of Bs among the worms of the laboratory cultures during long-term cultivation, the transmission rates of Bs in the progeny obtained from crosses of worms with different numbers of Bs, and from self-fertilized isolated worms. The DNA content of the Bs in M. mirumnovem was analyzed with the chromosomal in situ suppression (CISS) hybridization of microdissected DNA probes derived from A chromosomes (As). Bs mainly consisted of repetitive DNA. The cytogenetic analysis also revealed the divergence and high variation in large metacentric chromosomes (LMs) containing numerous regions enriched for repeats. The possible mechanisms of the appearance and evolution of Bs and LMs in species of the Macrostomum genus were also discussed.

Chromosome Evolution in the Genus Partamona (Apidae: Meliponini), with Comments on B Chromosome Origin

The genus <i>Partamona</i> includes 33 species of stingless bees, of which 11 were studied cytogenetically. The main goal of this study was to propose a hypothesis about chromosomal evolution in <i>Partamona</i> by combining molecular and cytogenetic data. Cytogenetic analyses were performed on 3 <i>Partamona</i> species. In addition, the molecular phylogeny included mitochondrial sequences of 11 species. Although the diploid number was constant within the genus, 2n = 34, B chromosomes were reported in 7 species. Cytogenetic data showed karyotypic variations related to chromosome morphology and the amount and distribution of heterochromatin and repetitive DNA. The molecular phylogenetic reconstruction corroborated the monophyly of the genus and separated the 2 clades (A and B). This separation was also observed in the cytogenetic data, in which species within each clade shared most of the cytogenetic characteristics. Furthermore, our data suggested that the B chromosome in the genus <i>Partamona</i> likely originated from a common ancestor of the species that have it in clade B and, through interspecific hybridization, it appeared only in <i>Partamona rustica</i> from clade A. Based on the above, <i>Partamona</i> is an interesting genus for further investigations using molecular mapping of B chromosomes as well as for broadening phylogenetic data.

High diversity of small insectivorous mammals on Qinghai–Tibet Plateau and first description of karyotype for four endemics of China

Among seven species of the order Eulipotyphla (from southern Gansu and northern Sichuan Provinces, Central China) studied cytogenetically, karyotypes of one talpid species, Uropsilus aff. soricipes (2n = 36, NFa = 54), and three soricid species, Chodsigoa hypsibia (2n = 65, NFa = 66), Sorex cansulus (2n = 42, NFa = 64) and Sorex thibetanus (2n = 42, NFa = 60), are described cytogenetically for the first time. All four species are endemic to China with distribution ranges restricted to the Qinghai–Tibet Plateau and adjacent mountain ranges. The Ch. hypsibia karyotype consists of mostly acrocentric autosomes and one metacentric pair of autosomes; besides, a B chromosome was identified. No polymorphism was detected among karyotypes of other species, including shrews Sorex bedfordiae (2n = 26, NFa = 44), Anourosorex squamipes (2n = 48, NFa = 92) and Crocidura suaveolens (2n = 40, NFa = 44). The Chinese shrew mole U. aff. soricipes and three shrew species (S. bedfordiae, Ch. hypsibia and A. squamipes) represent autochthonous fauna of Central/Western China, whereas S. thibetanus, S. cansulus and C. suaveolens belong to phylogenetic groups occurring mostly to the north and west from China; therefore, they should be considered relatively recent colonisers. Thus, considering the relationships of the species within phylogenetic groups, our results on karyotypes are in good agreement with molecular genetic data.

The supernumerary B chromosome of maize: drive and genomic conflict

The supernumerary B chromosome of maize is dispensable, containing no vital genes, and thus is variable in number and presence in lines of maize. In order to be maintained in populations, it has a drive mechanism consisting of nondisjunction at the pollen mitosis that produces the two sperm cells, and then the sperm with the two B chromosomes has a preference for fertilizing the egg as opposed to the central cell in the process of double fertilization. The sequence of the B chromosome coupled with B chromosomal aberrations has localized features involved with nondisjunction and preferential fertilization, which are present at the centromeric region. The predicted genes from the sequence have paralogues dispersed across all A chromosomes and have widely different divergence times suggesting that they have transposed to the B chromosome over evolutionary time followed by degradation or have been co-opted for the selfish functions of the supernumerary chromosome.

The B chromosome of Pseudococcus viburni: a selfish chromosome that exploits whole-genome meiotic drive

Meiosis, the key process underlying sexual reproduction, is generally a fair process: each chromosome has a 50% chance of being included into each gamete. However in some organisms meiosis has become highly aberrant with some chromosomes having a higher chance of making it into gametes than others. Yet why and how such systems evolve remains unclear. Here we study the unusual reproductive genetics of mealybugs, in which only maternal-origin chromosomes are included into the gametes during male meiosis, while paternally-derived chromosomes degrade. This whole genome meiotic drive occurs in all males and is evolutionarily conserved. However one species - the obscure mealybug Pseudococcus viburni - has a segregating B chromosome that increases in frequency by escaping paternal genome elimination. Here we present whole-genome and gene expression data from laboratory lines with and without B chromosomes. These data allow us to identify B-linked sequences including >70 protein-coding genes as well as a B-specific satellite repeat that makes up a significant proportion of the chromosome. We also used these data to investigate the evolutionary origin of the B chromosome. The few paralogs between the B and the core genome are distributed throughout the genome, showing that it is unlikely that the B originated through a simple duplication of one of the autosomes. We also find that while many of the B-linked genes do not have paralogs within the P.viburni genome, but they do show orthology with genes in other hemipteran insects suggesting that the B might have originated from fission of one of the autosomes, possibly followed by further translocations of individual genes. Finally in order to understand the mechanisms by which the B is able to escape elimination when paternally-derived we generated gene expression data for males and females with and without B chromosomes. We find that at the developmental stage when meiosis is taking place only a small number of B-linked genes show significant expression. Only one gene was significantly over-expressed during male meiosis, which is when the drive occurs: a acetyltransferase involved in H3K56Ac, which has a putative role in meiosis and is therefore a promising candidate for further studies. Together, these results form a promising foundation for studying the mechanisms of meiotic drive in a system that is uniquely suited for this approach.

High Diversity of Small Insectivorous Mammals on Qinghai-Tibet Plateau and First Description of Karyotype for four Endemics of China

Among seven species of the order Eulipotyphla (from southern Gansu and northern Sichuan Provinces, Central China) studied cytogenetically, karyotypes of one talpid species, Uropsilus aff. soricipes (2n = 36, NFa = 54), and three soricid species, Chodsigoa hypsibia (2n = 65, NFa = 66), Sorex cansulus (2n = 42, NFa = 64) and Sorex thibetanus (2n = 42, NFa = 60), are described cytogenetically for the first time. All four species are endemic to China with distribution ranges restricted to the Qinghai-Tibet Plateau and adjacent mountain ranges. The Ch. hypsibia karyotype consists of mostly acrocentric autosomes and one metacentric pair of autosomes; besides, a B chromosome was identified. No polymorphism was detected among karyotypes of other species, including shrews Sorex bedfordiae (2n = 26, NFa = 44), Anourosorex squamipes (2n = 48, NFa = 92) and Crocidura suaveolens (2n = 40, NFa = 44). The Chinese shrew mole U. aff. soricipes and three shrew species (S. bedfordiae, Ch. hypsibia and A. squamipes) represent autochthonous fauna of Central/Western China, whereas S. thibetanus, S. cansulus and C. suaveolens belong to phylogenetic groups occurring mostly to the north and west from China; therefore, they should be considered relatively recent colonisers. Thus, considering the relationships of the species within phylogenetic groups, our results on karyotypes are in good agreement with molecular genetic data.