Hybrids vs OP varieties [Was: Genetic engineering is a Good Thing?]

Ian Mackay mac at lionseed.demon.co.uk
Mon Sep 21 17:14:57 EST 1998


In article <msachs-2109981428580001 at marty7.agn.uiuc.edu>, Marty Sachs
<msachs at uiuc.edu> writes
>In article <0$rICJAbKeB2EwMO at upthorpe.demon.co.uk>, Oz
><Oz at upthorpe.demon.co.uk> wrote:
>
>> In article <A6589F7E4E3DD211AA7F0000C0E21BD57F0F29 at aricrmntnews.isd.dpi.
>> qld.gov.au>, Ian Staples <ianst at refer.to.signature.au> writes
>> 
>> >The advantage of hybrids is not necessarily simply yield _per se_.  
>> >A major factor is that hybrids derived from inbred lines are 
>> >genetically uniform, which usually results in a more uniform phenotype.
>> >As a result you get consistent quality, uniform ripening, easier and
>> >more efficient harvesting, and similar benefits.
>> 
>> Why was this a problem for maize, but not for wheat and barley (and most
>> crops in fact) that breed pretty true?
>
>
>Maize and its ancestor are naturally out-crossers (separate male and
>female flowers coupled with wind pollination), while wheat and barley and
>relatives are naturally selfers.  Maize is naturally very polymorphic and
>loses vigor as it becomes inbred.  With OPVs and land races, virtually
>every plant in a farmer's field is genetically different, so wind
>pollination maintains polymorphism and vigor in subsequent generations. 
>With hybrids, the farmer's field is, at best, limited to two alleles for
>each gene. So, each subsequent generation would be more and more inbred
>and there is very noticeable loss in yield after only one generation.

In fact, most of the loss comes in the first generation. As a first
approximation, the reduction in yield after the first generation of self
fertilization is half the total loss after many generations of selfing.

If  F2 seed (ie the seed harvested on the F1 hybrid plants) is grown on
and the F2 plants allowed to  open pollinate, then (assuming the plants
outcross), there will be no further reduction in vigour since there will
be no further increase in homozygosity.


>  It
>took special efforts to breed maize inbred lines that produced decently
>enough so that they could be used as economically viable parents of
>hybrids.  Wheat, barley, etc. were always selected as inbreds, and
>required no special breeding efforts to go from polymorhic lines to
>inbreds.  So, while there is somewhat of a heterotic boost for hybrid
>wheat, it is not as significant as in maize.

I once spent 6 months (about 10 years ago) analysing hybrid wheat data
for my then employer. The average level of advantage over inbred
varieties was about 8%. In my final report I wrote that this was "rather
disappointing". The *!#&£$ to whom I reported told me off for using
emotive language! In fact that was the only comment the *!*% made. And
am I bitter? Noooooooo.


-- 
Ian Mackay



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