Unique sequence organization and small RNA expression of a "selfish" B chromosome.

B chromosomes are found in numerous plants and animals. These nonessential, supernumerary chromosomes are often composed primarily of noncoding DNA repeats similar to those found within transcriptionally "silenced" heterochromatin. In order to persist within their resident genomes, many B chromosomes exhibit exceptional cellular behaviors, including asymmetric segregation into gametes and induction of genome elimination during early development. An important goal in understanding these behaviors is to identify unique B chromosome sequences and characterize their transcriptional contributions. We investigated these properties by examining a paternally transmitted B chromosome known as paternal sex ratio (PSR), which is present in natural populations of the jewel wasp Nasonia vitripennis. To facilitate its own transmission, PSR severely biases the sex ratio by disrupting early chromatin remodeling processes. Through cytological mapping and other approaches, we identified multiple DNA repeats unique to PSR, as well as those found on the A chromosomes, suggesting that PSR arose through a merger of sequences from both within and outside the N. vitripennis genome. The majority of PSR-specific repeats are interspersed among each other across PSR's long arm, in contrast with the distinct "blocks" observed in other organisms' heterochromatin. Through transcriptional profiling, we identified a subset of repeat-associated, small RNAs expressed by PSR, most of which map to a single PSR-specific repeat. These RNAs are expressed at much higher levels than those arising from A chromosome-linked repeats, suggesting that in addition to its sequence organization, PSR's transcriptional properties differ substantially from the pericentromeric regions of the normal chromosomes.

Spermatogenesis in haploid males of the jewel wasp Nasonia vitripennis.

Males of hymenopteran insects, which include ants, bees and wasps, develop as haploids from unfertilized eggs. In order to accommodate their lack of homologous chromosome pairs, some hymenopterans such as the honeybee have been shown to produce haploid sperm through an abortive meiosis. We employed microscopic approaches to visualize landmark aspects of spermatogenesis in the jewel wasp Nasonia vitripennis, a model for hymenopteran reproduction and development. Our work demonstrates that N. vitripennis, like other examined hymenopterans, exhibits characteristics indicative of an abortive meiosis, including slight enlargement of spermatocytes preceding meiotic initiation. However, we saw no evidence of cytoplasmic buds containing centrioles that are produced from the first abortive meiotic division, which occurs in the honeybee. In contrast to other previously studied hymenopterans, N. vitripennis males produce sperm in bundles that vary widely from 16 to over 200, thus reflecting a range of cellular divisions. Our results highlight interesting variations in spermatogenesis among the hymenopteran insects, and together with previous studies, they suggest a pattern of progression from meiosis to a more mitotic state in producing sperm.