|
The 'FecB' mutation has probably been present in Australia since 1792 but to date has had little impact on the local commercial sheep industry. This is despite considerable research efforts, which commenced in the late 1950s. In Merino sheep the combination of poor lamb survival of twins and triplets under extensive Australian management systems and the comparatively lower economic incentives to boost reproductive rate have been the major impediments to commercial success. The gene has also been introgressed into meat breeds, but competition from traditional or novel alternative breeds or crosses that also exhibit high fecundity has limited the impact of 'FecB'. The development of the direct DNA test for 'FecB' genotyping in 2001 was a major advance that provided a modest rekindling of research activity in Australia. Recent work with DNA genotyped sheep in a commercial Merino flock has confirmed many of the early findings in the Booroola Merino, but has also demonstrated a previously unreported 'homozygote penalty'. On the New South Wales property 'Allandale', homozygote carriers of 'FecB' exhibit significantly lower conception rate and lamb survival rates than non-carriers, something not observed in the heterozygote. It is postulated that this is due to uterine effects associated with excessive ovulation rate in homozygotes. Attempts to modulate litter size and lamb survival by restricting nutrition at mating time have proved unsuccessful in Merinos, although there are unpublished reports of successful modulation of these traits in a new meat composite breed carrying 'FecB'. The comparatively lower ovulation rates and litter sizes of 'FecB'-carrier ewes observed in Deccani crosses in India is encouraging as it suggests that there are environmental and/or genetic mechanisms which modulate 'FecB' expression. If these can be identified and effectively exploited, the future may yet be bright for the 'FecB' in Australian Merino sheep. However, on the basis of long experience, cautious skepticism is warranted. |
|