Potential for Genetic Improvement of Resistance to Perkinsus olseni in the Manila Clam, Ruditapes philippinarum, Using DNA Parentage Assignment and Mass Spawning

The Manila clam Ruditapes philippinarum, a major cultured shellfish species, is threatened by infection with the microparasite Perkinsus olseni, whose prevalence increases with high water temperatures. Under the current trend of climate change, the already severe effects of this parasitic infection might rapidly increase the frequency of mass mortality events. Treating infectious diseases in bivalves is notoriously problematic, therefore selective breeding for resistance represents a key strategy for mitigating the negative impact of pathogens. A crucial step in initiating selective breeding is the estimation of genetic parameters for traits of interest, which relies on the ability to record parentage and accurate phenotypes in a large number of individuals. Here, to estimate the heritability of resistance against P. olseni, a field experiment mirroring conditions in industrial clam production was set up, a genomic tool was developed for parentage assignment, and parasite load was determined through quantitative PCR. A mixed-family cohort of potentially 1,479 clam families was produced in a hatchery by mass spawning of 53 dams and 57 sires. The progenies were seeded in a commercial clam production area in the Venice lagoon, Italy, where high prevalence of P. olseni had previously been reported. Growth and parasite load were monitored every month and, after 1 year, more than 1,000 individuals were collected for DNA samples and phenotype recording. A pooled sequencing approach was carried out using DNA samples from the hatchery broodstock and from a Venice lagoon clam population, providing candidate markers used to develop a 245-SNP panel. Parentage assignment for 246 F1 individuals showed sire and dam representation were high (75 and 85%, respectively), indicating a very limited risk of inbreeding. Moderate heritability (0.23 ± 0.11-0.35 ± 0.13) was estimated for growth traits (shell length, shell weight, total weight), while parasite load showed high heritability, estimated at 0.51 ± 0.20. No significant genetic correlations were found between growth-associated traits and parasite load. Overall, the preliminary results provided by this study show high potential for selecting clams resistant to parasite load. Breeding for resistance may help limit the negative effects of climate change on clam production, as the prevalence of the parasite is predicted to increase under a future scenario of higher temperatures. Finally, the limited genetic correlation between resistance and growth suggests that breeding programs could incorporate dual selection without negative interactions.

In vitro and in vivo comparison of Ham's F-10, Emcare holding solution and ViGro holding plus for the cooled storage of equine embryos

Equine embryos have been successfully transferred after 24h cooled storage in Ham's F-10. The aim of this study was to compare the viability of equine embryos in vitro and in vivo after 6 and 24h cooled storage using three media and to examine the relationship between embryo size and viability after 24h cooled storage. In Experiment 1, the viability of embryos was evaluated using DAPI-staining after 0, 6 or 24h in Ham's F-10, 24h in Emcare embryo holding solution (EHS) or 24h in ViGro holding plus (VHP) (n=10/group). The mean number of dead cells was similar for embryos stored in Ham's F-10, EHS and VHP for 24h. Larger Day 7 embryos appear to withstand 24h cold storage better than small Day 7 embryos. The embryo quality for 24h cold storage was negatively correlated with size. In Experiment 2, 40 embryos were stored (n=20/group) either in Ham's F-10 or in EHS then transferred as pairs in recipient mares. Fifteen of the 20 recipient mares (75%) were pregnant. Out of 17 surviving embryos, 9 embryos (53%) were stored in Ham's F-10 and 8 (47%) in EHS. These results suggest that EHS and VHP offer a good alternative to Ham's F-10 for 24h cooled storage of equine embryos and that larger embryos may have a better viability after 24h of cooled storage than smaller embryos.

Which insemination results in fertilization when several are performed before ovulation?

The aim of the present study was to determine which artificial insemination results in fertilization when mares are inseminated several times before ovulation. Mares in oestrus were inseminated over 62 cycles with fresh semen at 48 h intervals from when a follicle > or =30 mm in diameter was detected until ovulation. The number of inseminations was limited to three. Three fertile stallions were used and a different stallion was used for each artificial insemination. The order of the three stallions was changed for each cycle. Embryos were collected between day 10 and day 12 after ovulation and paternity was checked using DNA typing. When two inseminations were performed per cycle, 14 of 17 embryos were the result of the insemination performed on days 2-4 before ovulation and three embryos were the result of the insemination performed on days 0-2. When three inseminations were performed, 1 of 6, 2 of 6 and 3 of 6 embryos resulted from the inseminations performed 4-6, 2-4 and 0-2 days before ovulation, respectively. Thus, 17 of 23 (74%) oocytes were not fertilized as a result of the insemination performed 0-2 days before ovulation. The mean interval between fertilization and ovulation in the mares from which embryos were recovered and tested (n=23) was 2.6 +/- 1.0 days. These results indicate that spermatozoa can remain viable in the genital tract of mares for at least 2.6 days.

Parentage testing of Day 10 equine embryos by amplified PCR analysis of microsatellites

Paternity analysis was performed on the DNA of 21 equine embryos collected nonsurgically 10 days after ovulation from known mares, but involving 3 possible sires. After extraction, the DNA of each embryo was typed by radioactive PCR amplification using 10 characterised microsatellites; HMS 1, 2, 5, 6, 7 and 8 (Guérin et al. 1994) and HTG 3, 4, 6 and 10 (Marklund et al. 1994). The 21 dams and 3 sires were genotyped using DNA extracted from blood and amplified by PCR. After electrophoresis and autoradiography of the PCR products of the embryo and parents, the alleles of the embryo were compared to those of the dam to identify those of maternal origin. The paternal alleles were then searched for within the genotype of the 3 sires, and the stallion(s) that exhibited the particular allele was said to be compatible with the embryo for this microsatellite. In this way, the true sire was identified correctly for all 21 embryos.

Follicular fluid is not a compulsory carrier of the oocyte at ovulation in the mare

The aim of this study was to test the possibility that ovulation can occur from a preovulatory follicle emptied of its follicular fluid. Transport of the oocyte into the oviduct and fertilisation in 29% of cases demonstrated that ovulation can occur in the absence of follicular fluid but the higher fertility achieved in control mares (62.5%) suggested that follicular fluid does serve a role during ovulation, fertilisation and oviductal transport. Injection of horse oocytes into preovulatory follicles in mules after removal of the follicular fluid, followed by insemination of the mules with horse semen, resulted in the production of one horse x horse embryo.