marker-assisted selection in wheat - ictsd

Chapter 13 – Marker-assisted selection in sheep and goats 235the end of 2005, a new single system for anational across-flock genetic evaluation ofMerinos had been introduced in Australia,combining data from previously separateschemes. The number of animals performancerecorded per year is growing rapidly,with about 100 000 new animals now beingentered annually.Wool production efficiency is mainlydetermined by fleece weight and woolquality. Wool quality traits are mainly fibrediameter and staple strength, and these areeconomically much more important for finewools. Staple strength is more expensive tomeasure, but has a high correlation with thecoefficient of variation of fibre diameter,which is therefore a good predictor. Wooltraits have generally high levels of heritability,especially fleece weight and fibrediameter.Reproductive rate in wool sheep hasbeen hard to select for as pedigree recordinghas been limited and the heritability islow. Moreover, genetic improvement ofreproductive rate has been less importantfor wool production because of thepositive net economic benefit of wool producingbreeding females. However, with anincreasing meat/wool price ratio, the situationis changing and reproductive rate iscurrently becoming more important. Also,meat attributes of Merino sheep are nowreceiving increased attention, includingmeasurements of body weight at differentages, fat depth and eye muscle depth (ultrasoundscanned).In pure wool production systems, MASwould be expected to have limited benefitfor wool production traits because of theirhigh heritability and the ability to measurethe traits before the age of first selection.MAS for reproductive traits and motheringability would be more beneficial because oflow heritability and sex-limited recording.Parasite resistance is becoming a trait ofgreater economic importance due to thedevelopment of resistance to all the majorclasses of anthelmintics used and the lackof new anthelmintic classes being developed.Host resistance to internal parasitesis particularly poor in the Merino breed.The trait can be selected for using fieldrecords of WEC. EBVs are being producedfor this trait and genetic progress is beingachieved. However, the procedure is laboriousand there is also some concern aboutuniformity of measurement and trait definition,as well as the existence of differentspecies of parasites in different regions.Various studies have looked at genotypex environment interactions for parasiteresistance and, although some interactionexists, relatively high correlations (~0.8)were found between breeding values in differentenvironments, when environmentswere defined either through worm type(McEwan et al., 1997) or by high and lowflock averages for WEC (Pollot and Greeff,2004). In any case, many of these traitattributes make parasite resistance a goodtarget for MAS. Identifying QTL for parasiteresistance might also shed more lighton the biology of immunity, and possiblyhelp to find other modes of improvement.Feed efficiency, and particularly maternalefficiency, are important determinants ofpastoral production systems (Ferrell andJenkins, 1984) and genetic improvementwould benefit from MAS because of thecost of their measurement. However, feedavailability and feed costs are quite variablewithin and between years, and the abilityof sheep to cope with harsh environmentsand periods of drought is perceived byindustry as being of greater importance.Hardiness and ewe survival are not welldefined characteristics and are not normallymeasured in breeding programmes.