BIOTECH-ROOM3-L Archives

Moderated conference on Genomics in Food and Agriculture

Biotech-Room3-L@LISTSERV.FAO.ORG

Options: Use Forum View

Use Monospaced Font
Show Text Part by Default
Condense Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Content-Type:
text/plain; charset="iso-8859-1"
Date:
Mon, 11 Mar 2013 13:56:39 +0100
Reply-To:
Biotech-Mod3 <[log in to unmask]>
Subject:
Content-Transfer-Encoding:
quoted-printable
Message-ID:
MIME-Version:
1.0
Sender:
Moderated conference on Genomics in Food and Agriculture <[log in to unmask]>
From:
Biotech-Mod3 <[log in to unmask]>
Parts/Attachments:
text/plain (38 lines)
This is Uche Godfrey Okeke, again. Greetings again to everyone and special thanks to everyone contributing in these discussions.

I am very pleased with the statement from John Gibson (Message 17) stating: " genetic improvement is too often based on wishful thinking rather than critical assessment of farmer livelihoods, needs and demands......research geneticists have a poor track record in both developed and developing world of producing products that farmers actually want". 

This is exactly where the focus should be. It should be the rural community, the society and the local market niche that should tell us what to breed. If this is adhered to, then there is hope for food security. Just as Paulo Ramos (Message 15) described, regional breeding will give power to the rural people and not corporations. This is where I am very hopeful. The use of genomic selection technology promises gains in short time so that the gap we are missing in genetic improvement is filled within a short time period.

Secondly, I would like to talk more (see my Message 10) about selection-induced genetic variation (SIGV). 

Lynch and Walsh (1998) in their famous book "Genetics and analysis of quantitative traits" clearly outline epistasis as one major source of genetic variation. Epistasis is simply said to be interaction of genes between loci (i.e. interaction between genes from different loci). This interaction grows with increase in the number of loci. They clearly state that epistatic interactions greatly inflate additive and dominance variances. It follows that even if epistatic effects are small, their summed effects may be very large. In fact, all genetic variation due to non-additive gene actions is by epistasis (Lynch & Walsh, 1998). For example given two loci, epistatic interactions can be additive X additive, additive X dominance or dominance X dominance. Remember that these interactions increase in power (^) with increased number of loci.

According to Eitan and Soller (2004), consider quantitative trait loci (QTLs) in a population under selection. These QTLs (or alleles or genes with effect on the complex trait) have additive or summed effect but they also exhibit strong interactions between each other from different loci (epistatic interactions). Recall that this is a population so that these QTLs are from different genetic backgrounds.

Now, this implies that a QTL or allele or gene with a minor (small) or neutral (simply understand as no effect - but no gene has a zero effect) effect in genetic background A might have a strong effect when it is transferred to another genetic background B. With this, a neutral or minor locus transforms into a QTL in a favorable genetic background. This is because it is interacting with good match QTLs from different loci (by good match I mean favorable co-adapted gene complexes which brings out the best expression from individual QTLs in these complexes). 

Recall that selection fixes favorable QTLs (or alleles or genes) in a population. Now this fixation changes the genetic background of the QTLs segregating in the population. And if these backgrounds are changed, new epistatic interaction complexes are formed leading to novel gene expressions or trait values. Eitan and Soller even state that transfer of neutral alleles into new genetic backgrounds can transform these alleles into full fledged QTLs. With this, formerly neutral alleles move into additive action and these happen in an iterative manner (remember when I talked about epistasis in powers and influenced by number of participating loci) without any limit. A very interesting phrase from Eitan and Soller is that selection acts as a bootstrap (i.e. continuous sampling), continuously generating at least a part of the genetic variation required for its own continued response. Another vital point is that favorable pleiotropic effects will form desired and coordinated network of interactions that can develop into superior traits.

The key lessons are:
1. Selection introduces changes in the genetic background thereby introducing QTLs for epistatic interaction.
2. A previously neutral locus can be transformed into the most desired QTL in a different genetic background.

Now, if you think about heterosis or hybrid vigor. According to Lynch and Walsh (1998), heterosis is a result of favorable or positive dominance and additive X additive epistasis. As long as there is a build-up of favorable epistatic complexes, heterosis will occur.

I have tried to think about SIGV model as heterosis (per se) in a population under selection. As long as there is build-up of favorable epistatic complexes, there will be enough genetic variation to drive selection forward.

Uche Godfrey Okeke.
DNA sequencing & Genomics Lab
Institute of Biotech
University of Helsinki, 
Finland.
Email: urchgene (at) gmail.com , urchgene (at) yahoo.com

[To contribute to this conference, send your message to [log in to unmask] For further information on this FAO Biotechnology Forum, see http://www.fao.org/biotech/biotech-forum/ ]

########################################################################

To unsubscribe from the Biotech-Room3-L list, click the following link:
https://listserv.fao.org/cgi-bin/wa?SUBED1=Biotech-Room3-L&A=1

ATOM RSS1 RSS2