Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities

Therian mammals have among the oldest and most conserved sex-determining systems known to date. Any deviation from the standard XX/XY mammalian sex chromosome constitution usually leads to sterility or poor fertility, due to the high differentiation and specialization of the X and Y chromosomes. Nevertheless, a handful of rodents harbor so-called unusual sex-determining systems. While in some species, fertile XY females are found, some others have completely lost their Y chromosome. These atypical species have fascinated researchers for over 60 years, and constitute unique natural models for the study of fundamental processes involved in sex determination in mammals and vertebrates. In this article, we review current knowledge of these species, discuss their similarities and differences, and attempt to expose how the study of their exceptional sex-determining systems can further our understanding of general processes involved in sex chromosome and sex determination evolution.

P–581 The day of blastocyst biopsy and the chromosomal constitution. Evaluation of 5125 embryos by Next Generation Sequency (NGS)

Study question Is there a relationship between the day of blastocyst biopsy and the results NGS analysis? Summary answer Embryos biopsied on day 6 or 7 are associated with the increased probability of being an aneuploidy embryo and less likely to be mosaic embryo. What is known already There is controversy about whether an embryo that reaches the blastocyst stage on day 5 has a higher chance of being euploid than embryos which are biopsied later. In our study, chromosome constitution was evaluated by next-generation sequencing (NGS)-based preimplantation genetic testing for aneuploidy (PGT-A) and confounding factors were eliminated. Study design, size, duration Data was collected retrospectively from June 2016 to January 2020 Participants/materials, setting, methods In total, 5125 blastocyst (day 5=2914, day 6 N = 2154 and day7 N = 57), generated from 1318 cycles were analysed with PGT-A. The chromosome constitution for each embryo was classified as euploid, aneuploid and mosaic. A multilevel model was made and associations betwwen variables by logistic regression were adjusted according to maternal age, SART blastocyst grade, fertilization method, biopsy operator and blastocyst stage. Main results and the role of chance The mean maternal age was 36.2 ± 4.2. Euploid rate was 62.1% and 37.9% (day 5 and day 6–7 respectively), aneuploidy rate was 47.0% and 53.0% (day 5 and day 6–7, respectively), mosaicism rate was 59.6% and 40.4% (day 5 and day 6–7, respectively) (p < 0.001). Embryos biopsied on day 6–7 have a significantly lower probability to be euploid and mosaicism than embryos biopsied on day 5 ((OR = 0.76 [0.68–0.86]); (OR = 0.84 (0.73 – 0.96) respectively) (p < 0.001). On the contrary, embryos biopsy on day 5 were significantly more likely to be euploid than day 6–7 (OR = 1.63[1.42–1.86]) (p < 0.001). Limitations, reasons for caution The results observed in this study should be confirmed using a larger number of samples. For the NGS analysis, a chromosome with a variation between 20 to 80% was considered mosaic. Wider implications of the findings: The present study revealed that embryos that reach blastocyst classified as full to hatched on day 5 are more like to be euploid compared to slow growing embryos. Trial registration number non-clinical trials

Evolution of the Sex Chromosomes in Beetles. I. The Loss of the Y Chromosome

In the males of Coleoptera, the most frequent sex chromosome constitution is XY. At metaphase I of meiosis, the X and Y are linked by nucleolar proteins, forming the so-called parachute bivalent (Xyp), which is assumed to allow the non-synapsed X and Y to segregate correctly at anaphase I. However, X0 males are not exceptional, and we explored the relationships between the X and nucleolar proteins in the absence of the Y chromosome in 6 species belonging to different families/subfamilies. Using C-banding and silver staining, we show that nucleolar proteins always remain in contact with the X until anaphase I. These proteins are generally more abundant than in the Xyp bivalent, may remain associated with the NOR during diakinesis, and frequently link the X to 1 or 2 autosomal bivalents, which seem to play the same role as the Y. This role may also be played by B chromosomes, which appear to be more frequent in X0 than in XY males. In conclusion, following Y chromosome loss, various strategies using nucleolar proteins have been developed to facilitate the migration of the unique X at meiotic anaphase I.