19 August 2013

Another late winter update - The Coloured Pea Project selection strategies

While I was away on leave, I had Craig, a grad student of mine, come in and assess the autumn pea growout, and collect pods from any coloured peas to dry down. Returned to find lots of paper lunch bags all neatly labelled and drying in the shed, but it smelled a bit musty, so all the bags came in and were dried off for a couple of days in the oven set to 'defrost'.

Production was poor - we had a cold wet winter for Bendigo, and I think the Powdery Mildew might have shortened the lives of many plants.

But I got lots of very promising lines to explore.

Last autumn I went through my purple podded project growouts, which essentially came from three crossed lines

Delta Louisa X Purple Podded - target = a semi-dwarf semi-resistant snow
Tall Purple-flowered Mammoth X Purple Podded - target = a tall non resistant big podded snow
Chamber of Death X Purple Podded - target = a highly resistant semi-dwarf snow

I was also looking at developing a better yellow snow, by crossing Golden Podded with my Purple Flowered Mammoth.

(There were a number of other crosses - a generic sugar snap X Purple Podded, Golden Podded X Purple Podded, and a few other random crosses because I could).
The F2 grow outs from last spring yielded a number of promising lines, half purple snows, full purple big podded non-snows, disease resistant half purple dwarf snows, disease resistant tall purple non-snows, seemingly every combination of characteristics, apart from the target . So how to proceed?

I set up 5 'projects' using the F3 seed material I had harvested.

TPM - Tall Purple Mammoth, looking for a big podded purple snow, with no disease resistance
TMM - Tall Mauve Mammoth looking for a big-podded half purple snow, no disease resistance
RPC - Resistant Purple Chamber of Death - looking for a highly resistant purple snow, tall or short
RMC - Resistant Mauve Chamber of Death - looking for a highly resistant half purple snow, tall or short.
 TYS - Tall Yellow Snow - looking for a tall yellow snow, with big pods and good flavour, with no disease resistance.

I thought I could persue these lines, using selection and even if I didn't get the desired target, I could use the mauve lines to back cross or cross together if they had the desirable characteristics. Rather than plant a few of everything, I selected the most promising parents, and grew out 6 seeds from 12 of the lines in foam boxes in the shade through the middle of summer - didn't really work, with only one or two lines producing any seed. So in autumn, I resowed a lot of the F3 seed, and a couple of F4 mauves that had done OK (plant 6).

So now I have a pile of F4 seed from the autumn growouts, but most plants only produced one or two small pods. This is disappointing on a number of fronts - I didn't get to test the quality of the pods, and there isn't really enough material to do big selected F4 growouts to check on homo/heterozygosity.

Dividing the lines early into Purple and Mauve projects was a bit premature - the purple lines produced sufficient mauve material in the F3 plants to go on with, but suprisingly, some of the mauve lines produced nearly full purples. This is a suprising result since pod colour is supposed to be controlled by 3 dominant genes, but it sure looks like there are other things going on here, with the purple sometimes being expressed in the later pods on a plant, but not on the early ones, and a number of different patterns of semi purple expression.

Another problem is the sheer amount of different material that is produced. I now have thousands of seeds in four generations across dozens of lines - just cataloging and storing it all takes so much time. And then making decisions about which lines to persue in my limited garden space - whew!

On the long drive between Halls Creek and Alice Springs and points further south, I spent a couple of days working out the probability of homozygous recessive genes being expressed in say an F3 generation if the F2 parent is heterozygous. My question was "How many plants would I need to grow of say an F3 generation to tell if the F2 parent was heterozygous at one locus?"  An example might help. Say I've got a tall pea, that had tall and dwarf characteristics in its parentage. It might be homozygous tall, that is carrying TT, so it will never give dwarf offspring. But what if it is heterozygous Tt? If I grow one plant, there is a 1/4 chance of it being homozygous dwarf, or tt. If I grow two plants the chances of at least one dwarf plant showing up, and therefore indicating the parent was heterozygous, is 1/4 + 3/16 = .4375. Three plants gives 37/64, or .578. I had forgotten my binomial distributions, so basically went back to first principles to figure it out.  I seem to recall that I came up with 6 plants to be around 82% sure, which doesn't meet the requirements of an ecology experiment, say, but will do me in my garden.

So how does this help me? If I want to find a purple snow, I can grow out 6 from a purple non-snow parent to see if one of the low fibre recessive genes is lurking there. This will let me know with about 80% confidence, whether the line is worth continuing on with or not. If it has no snow pea genes in it, then I can save time and space by rejecting those lines that are unlikely to produce what I want.

So I'm going with a combination of selection and mass grow out - growing 6 or so plants from all the purple lines, all of the seeds in the lines where purple is apparent but snow is also present, and a few 'hedge my bets' lines, that might prove to be promising parents for some characteristics if the purple growouts are not too succesful and I need to cross back to something for desirable characteristics. Pity that such a cross would need another couple of generations to roduce results, but hey, that's plant breeding!
No pics at the moment, but more to follow

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