Phenotypic, Genomic and Physiological Basis of Fecundity Traits in Bonga Sheep of Ethiopia
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Asrat Dolebo. (24/7/2020). Phenotypic, Genomic and Physiological Basis of Fecundity Traits in Bonga Sheep of Ethiopia.
Abstract
Three interrelated investigations were carried out with the objectives of estimating genetic
parameters and trends of selection for Age at First Lambing (AFL), Lambing Interval (LI) and
Litter size (LS); understanding the ovarian and endocrine changes and identifying the genetic
basis of prolificacy in Bonga sheep, Ethiopia. Ten years data (2009-2018) on reproductive
performance of Bonga sheep, managed by two communities involved in a community-based
breeding program were used for genetic parameter and trend analysis. Data on the reproductive
traits were analysed to evaluate the effects of breeding communities, season of mating, year of
lambing, and parity by fitting fixed effect model of GLM procedures of SAS. Restricted maximum
likelihood procedure of WOMBAT fitting univariate animal model was employed to estimate
heritability, repeatability and breeding values. Thirty-one ewes were selected based on LS records
and divided into two groups of high prolificacy (n=20) with LS ≥ 2 and low prolificacy (n=11)
with LS =1. At a synchronized oestrus, follicular dynamics were determined using transrectal
ultrasonography while plasma oestradiol concentrations were monitored throughout the induced
follicular phase. Whole blood was collected from 95 animals (31 gave birth to single lambs, 33 to
twins, 30 to triplets and one to a quadruplet) for genome analysis. Candidate regions under
selection were identified using selection signature analysis performed on Ovine HD BeadChip
data. Results showed that Bonga sheep had overall mean AFL, LI and LS of 453 days, 254 days
and 1.43 lambs, respectively. Estimates of heritability for AFL, LI and LS were 0.015, 0.009 and
0.085, respectively. The repeatability estimates for LI and LS were low (0.109 and 0.196,
respectively) indicating that environmental factors had contributed to the variation in these traits
among parities. The genetic trends for AFL, LI and LS over the years were significant (p<0.01).
Investigation of ovarian basis for prolificacy of Bonga sheep indicated that the mean number of
large follicles was higher (p < 0.05) in HP (high prolific), 1.78 than in LP (low prolific), 1.0 ewes
at day of oestrus (day 0). Prior to oestrus, more (p < 0.05) medium follicles were visible for HP
compared to LP ewes. Plasma oestradiol concentrations were higher in HP compared to LP ewes
(18.9 vs. 14.5 pg/ml; p < 0.05). Similarly, ovulation number was higher for HP than LP ewes (2.3
versus 1.28; p < 0.05) indicating that higher ovulation rates and litter size in Bonga sheep are
evidenced by the previous presence of more large follicles and the existence of co-dominance
effects. Analysis of selection signature revealed one strong selection signature on a candidate
region on chromosome X spanning BMP15, suggesting this to be the primary candidate prolificacy
gene in the breed. Besides, the analysis also identified several candidate regions spanning genes
not reported before in prolific sheep but underlying fertility, immunity and reproduction in other
species. The genes associated with female reproduction traits included SPOCK1 (age at first
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oestrus), GPR173 (mediator of ovarian cyclicity), HB-EGF (signalling early pregnancy success)
and, SMARCAL1 and HMGN3a (regulate gene expression during embryogenesis). The genes
involved in male reproduction were FOXJ1 (sperm function and successful fertilization) and
NME5 (spermatogenesis). It has been also observed that genes such as PKD2L2, MAGED1 and
KDM3B within the candidate regions, which might have been associated with diverse fertility
traits in males and females of other species. The results further confirmed the complexity of the
genetic mechanisms underlying reproduction while suggesting that prolificacy in the Bonga sheep
and possibly African indigenous sheep is partly under the control of BMP15 while other genes
that enhance male and female fertility are essential for reproduction fitness. It is concluded that the well-structured community-based breeding programs of Bonga sheep have resulted in
measurable genetic gains for reproductive traits. Besides, the existence of mutation is the causative
effect for the phenotypic observed differences in growth of follicles and variability in litter size in
Bonga sheep.
