Rederivation by Cryopreservation of a Paternal Line of Rabbits Suggests Exhaustion of Selection for Post-Weaning Daily Weight Gain after 37 Generations

Rabbit selection programmes have mainly been evaluated using unselected or divergently selected populations, or populations rederived from cryopreserved embryos after a reduced number of generations. Nevertheless, unselected and divergent populations do not avoid genetic drift, while rederived animals seem to influence phenotypic traits such as birth and adult weights or prolificacy. The study aimed to evaluate the effect of a long-term selection for post-weaning average daily weight gain (ADG) over 37 generations with two rederived populations. Specifically, two coetaneous populations were derived from vitrified embryos with 18 generational intervals (R19 and R37), reducing or avoiding genetic drift and environmental and cryopreservation effects. After two generations of both rederived populations (R21 vs. R39 generations), all evaluated traits showed some progress as a result of the selection, the response being 0.113 g/day by generation. This response does not seem to affect the estimated Gompertz growth curve parameters in terms of the day, the weight at the inflexion point or the adult weight. Moreover, a sexual dimorphism favouring females was observed in this paternal line. Results demonstrated that the selection programme had improved ADG without variations in adult body weight but, after 37 generations of selection, this trait seems exhausted. Given the reduction in the cumulative reproductive performance and as a consequence in the selection pressure, or possibly/perhaps due to an unexpected effect, rederivation could be the cause of this weak selection response observed from generation 18 onwards.

Long-term implications of feed energy source in different genetic types of reproductive rabbit females: I. Resource acquisition and allocation

To achieve functional but also productive females, we hypothesised that it is possible to modulate acquisition and allocation of animals from different genetic types by varying the main energy source of the diet. To test this hypothesis, we used 203 rabbit females belonging to three genetic types: H (n=66), a maternal line characterised by hyper-prolificacy; LP (n=67), a maternal line characterised by functional hyper-longevity; R (n=79), a paternal line characterised by growth rate. Females were fed with two isoenergetic and isoprotein diets differing in energy source: animal fat (AF) enhancing milk yield; cereal starch (CS) promoting body reserves recovery. Feed intake, weight, perirenal fat thickness (PFT), milk yield and blood traits were controlled during five consecutive reproductive cycles (RCs). Females fed with CS presented higher PFT (+0.2 mm, P0.05), particularly for those fed with AF. Moreover, LP females fed with AF progressively increased PFT across the RC, whereas those fed with CS increased PFT during early lactation (+7.3%; P<0.05), but partially mobilised it during late lactation (-2.8%; P<0.05). Independently of the diet offered, LP females reached weaning with similar PFT. H females fed with either of the two diets followed a similar trajectory throughout the RC. For milk yield, the effect of energy source was almost constant during the whole experiment, except for the first RC of females from the maternal lines (H and LP). These females yielded +34.1% (P<0.05) when fed with CS during this period. Results from this work indicate that the resource acquisition capacity and allocation pattern of rabbit females is different for each genetic type. Moreover, it seems that by varying the main energy source of the diet it is possible to modulate acquisition and allocation of resources of the different genetic types. However, the response of each one depends on its priorities over time.

Feed restriction regime in a rabbit line selected for growth rate alters oocyte maturation manifested by alteration in MSY2 gene expression

Young rabbit females selected for growth rate may have nutritional needs, which may not be met with the common practice of feed restriction during rearing in commercial rabbit production. The aim of this study was to analyse whether two different feeding programmes: ad libitum or restricted (130 g/day) feeding, applied in young rabbit females for 1 month at the end of rearing, could modulate the origin of ovulation process and the quality of the oocytes. At 16 weeks of age, 34 females were randomly assigned to restricted or ad libitum feeding, maintaining these conditions for a month. Then, in an initial experiment, transcriptional profiling of hypothalamus-hypophysis tissue was performed to assess failure to ovulate. In the second experiment, the gene expression analysis of some candidate genes related to oocytes quality was performed. Our results demonstrated that neither of the two feeding programmes modified the transcription of hypothalamus-hypophysis tissue, while the only differences in MSYR expression were found in in vivo mature oocytes ready for successful fertilization. Specifically, MSYR was over-expressed in oocytes from females fed ad libitum. MSYR is one of the most abundant proteins in the oocyte and has proven to be a key regulator of maternal RNA transcription and translation. This finding suggests that MSYR gene is a promising gene in our understanding of the relationship between high growth rate and reproductive performance decline.