A new plant sex-linked gene with high sequence diversity and possible introgression of the X copy

We describe patterns of DNA sequence diversity in a newly identified sex-linked gene, SlX9/SlY9, in Silene latifolia (Caryophyllaceae). The copies on both sex chromosomes seem to be functional, and each maps close to the respective X- and Y-linked copy of another sex-linked gene pair, SlCypX/SlCypY. The Y-linked copy has low diversity, similar to what has been found for several other Y-linked genes in S. latifolia, and consistent with the theoretical expectations of hitch-hiking processes occurring on a non-recombining chromosome. However, SlX9 has higher diversity than other genes on the S. latifolia X chromosome. We evaluate the hypothesis of introgression from the closely related species S. dioica as an explanation for the high sequence diversity observed.     

Centromere organization and UU/V sex chromosome behavior in a liverwort

In 1917, sex chromosomes in plants were discovered in a liverwort with hetermorphic U and V chromosomes. Such heteromorphy is unexpected because, unlike the XY chromosomes in diploid-dominant plants, in haploid-dominant plants the female U and the male V chromosomes experience largely symmetrical potential recombination environments. Here we use molecular cytogenetics and super-resolution microscopy to study Frullania dilatata, a liverwort with one male and two female sex chromosomes. We applied a pipeline to Illumina sequences to detect abundant types of repetitive DNA and developed FISH probes to microscopically distinguish the sex chromosomes. We also determined the phenotypic population sex ratio because biased ratios have been reported from other liverworts with heteromorphic sex chromosomes. Populations had male-biased sex ratios. The sex chromosomes are monocentric, and of 14 probes studied (eight satellites, five transposable elements and one plastid region), four resulted in unique signals that differentiated the sex chromosomes from the autosomes and from each other. One FISH probe selectively marked the centromeres of both U chromosomes, so we could prove that during meiosis each U chromosome associates with one of the opposite telomeres of the V chromosome, resulting in a head-to-head trivalent. The similarity of the two U chromosomes to each other in size and in their centromere FISH signal positions points to their origin via a non-disjunction event (aneuploidy), which would fit with the general picture of sex chromosomes rarely crossing-over and being prone to suffer from non-disjunction.     

The SMC-family Wadjet complex protects bacteria from plasmid transformation by recognition and cleavage of closed-circular DNA

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Chimaerism detection in bovine twins,triplets and quadruplets using sex chromosome‐linked markers

The phenomenon of chimaerism occurs in the majority of cattle twin pregnancies. The objectives of this study were to develop a powerful diagnostic test for chimaerism in bovine male and female co‐twins using X and Y chromosome‐linked markers and to determine the extent of chimaerism in twins, triplets and quadruplets. We developed a multiplex PCR set of three polymorphic markers on chromosome X (DIK2865, DIK2283, AGLA257), where the presence of >1 and >2 alleles per marker is sufficient to prove chimaerism in males and females, respectively. In addition, a specific segment on chromosome Y (BOV97M) is included in the set to indicate chimaerism in females. Visualization of chimaeric alleles was best for DNA extracted from blood, fair for DNA from vaginal smears and failed for DNA extracted from hair. The power of chimaerism identification using this set of markers for DNA extracted from blood was calculated as 99% in males and virtually 100% in females. All females and males in heterosexual twins, triplets and quadruplets displayed evidence of a chimaeric allele in at least one and maximum of three of three X chromosome markers analysed. In addition, all females showed the presence of the BOV97M segment and were validated as chimaeric by the standard clinical diagnosis of impaired vaginal length. Quantitative PCR analysis of BOV97M copies in all twins vs. their sires showed a mean ratio of 45–68% in females and 39–49% in males, indicating a substantial symmetrical exchange of cells among all co‐twins. The proposed analysis of X and Y chromosome‐linked markers is advantageous to previous methods based on Y chromosome sequences only, because it detects chimaerism in both male and female co‐twins.     

Allometry and chromosomal speciation of the casiraguas Proechimys (Mammalia, Rodentia)

Morphometric characters (linear measurements) of the skull and mandible of the rodent Proechimys occurring in Venezuela and Columbia were investigated through multivariate techniques in order to describe static allometry and to relate allometric patterns to the recent evolution of these taxa. At least five species originated during the recent Pleistocene period in the upper Orinoco basin, all characterized by different karyotypes, one of them (P. guairae) being a Rassenkreis of karyomorphs. Species are characterized by unique static allometric patterns and the study of these patterns suggests a phylogenetic rather than an ecogenetic origin for these patterns; modifications in the shape of the skull and the mandible have originated during phyletic speciation and there is no apparent sign of functional adaptation.     

CENP-B-mediated DNA loops regulate activity and stability of human centromeres

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