Sunday, 3 February 2008

Pea breeding project: golden yellow sugarsnaps

F1 hybrid from my cross of Golden Sweet x Sugar Ann. They have pretty bicolour flowers which turn blue as they begin to fade.

Golden Sweet x Sugar Ann
I'm quite excited at the prospect of following this project, which is aiming to develop a beautiful sugarsnap pea with golden yellow pods. To my knowledge, there aren't any yellow sugarsnaps available. However I know this path has been trod before, by Dr Alan 'Shroom' Kapuler, who happens to be a hero of mine ... a pioneer of public domain plant breeding, such a rare thing in this era of Plant Variety Rights and gene patents, who has created open-pollinated versions of many popular commercial hybrids, and breeds vegetables for their nutritional value. The result of his experiment with yellow sugarsnaps was Opal Creek, which doesn't appear to be available at the moment, although there's a lovely and informative description of its creation in Carol Deppe's amazing book (pp. 130-135). I'm hoping I can create a yellow sugarsnap of my own, and I'm using one of the same varieties Alan used, the unique yellow podded mangetout Golden Sweet.

But funnily enough this project kind of started by accident.

I planted companion crops of Golden Sweet and Desiree for the purpose of crossing them, the first stage of the Real Seeds purple mangetout project which I'll be writing about separately. However, Golden Sweet turned out to be earlier-flowering than Desiree, by about 10 days. Rather than waste the first few Golden Sweet flowers (the best seeds generally come from the earliest flowers) I crossed them with whatever I had handy. The only other pea I had flowering in the garden at the time was the pint-sized sugarsnap Sugar Ann.

I think I hand-pollinated four flowers, which is not very many. Then it started chucking it down with rain for several days, so I couldn't do any more. By the time the rain stopped, the Sugar Ann had run out of flowers so there was no more pollen to be had. Then one of my four pollinated pods aborted and fell off. But the other three thrived to maturity. That gave me a few F1 hybrid seeds.

As they were the first of my 2007 crosses to mature (around the end of June), I decided I had time to squeeze another life cycle into the growing season, so I immediately replanted some of the seed in bog roll tubes. They grew like rockets.

I planted them outside in a 4ft high frame and they grew out of the top of it within a couple of weeks. They ended up about double that size and draped over the frame in a fold of luxurious foliage. The flowers were very pretty. One of Golden Sweet's peculiarities is that its flowers never fully open, although they go through spectacularly beautiful colour changes in their half-open state. This hybrid looked to all intents and purposes like Golden Sweet, but had fully opening flowers. It's likely there's a recessive gene involved somewhere in that, in which case I'll get a few non-openers in the next generation.

Talking of recessive genes, it's been well established right back to Gregor Mendel's experiments in the 1850s that tall genes are dominant in peas and dwarf traits are recessive. In most cases, if you cross a tall pea with a dwarf one, all the F1 offspring will come out tall. Then if you replant their seeds (F2) you should get about a quarter dwarf and three-quarters tall ... what's known as a Mendelian ratio. My hybrids have conformed perfectly so far. They were sired by a tiny dwarf pea barely more than a foot high, but they soared into huge plants. A bit of hybrid vigour made them even taller than their tall parent.

Another thing that's recessive, as I read in Carol Deppe's book and now know from experience, is the yellow colouring. I've made several hybrids with Golden Sweet now, and none of them show any trace of yellow at the F1 stage. Again the Mendelian ratio should apply, so I can probably expect the next generation from this cross to be a quarter yellow and three quarters green.

Then we have the sugarsnap edible pods. Edible podded peas have two recessive genes which (in combination) stop the pod from forming an inedible inner membrane. Both Golden Sweet and Sugar Ann are edible podded, so I assume they both already have this genetic combination. So far so good. Sugarsnap peas additionally have a gene which makes the walls of the pod thick and juicy. This is also recessive. So I will expect most or all of the F2 offspring to have edible pods (as both parents are of that type) and roughly a quarter will be sugarsnaps and the rest mangetout, or snow peas.

What I'm chasing here is a double recessive class. If we call the dominant green-podded gene 'G' and the recessive yellow-podded gene 'g', and the flat-pod 'F' and the puffy (sugarsnap) pod 'f', this is what we get.

Sugar Ann is genotype GGff - green and puffy pods
Golden Sweet is genotype ggFF - yellow and flat pods

The genotype for yellow puffy pods would be ggff. Two pairs of recessive genes. That's the combination I'm looking for.

When I crossed the two original varieties, the F1 hybrid will have got half its genome from each parent, so their genotype is most likely GgFf. I therefore expected all the F1 plants to have green pods, since green is the dominant gene (the yellow gene is still in there, but doesn't express itself in the presence of a dominant gene). I also expected the pods to be flat, mangetout types, as the dominant flat-podded gene will mask the presence of the recessive sugarsnap gene. And that's exactly what I've ended up with, so that's a good start.

From my F1 plants I collected lots of seed (just by letting them self-pollinate, as peas are entirely self-fertile). These are the F2 seeds, which I will be planting in about a month's time, and somewhere among them there should be some yellow sugarsnaps. About one-sixteenth of the crop, in fact. Here's why: I have a one in four chance of getting the recessive yellow pod colour, because there are four possible gene combinations: GG (green), Gg (green), gG (green), or gg (yellow). I also have a one in four chance of getting puffy sugarsnap pods, as the F2 seeds may carry FF (flat), Ff (flat), fF (flat) or ff (puffy). Multiply those possible combinations together ... and my chance of getting genotype ggff, both pairs of recessives in one plant, is one in sixteen.

These are of course just statistics showing the probabilities, and the actual numbers could vary either way.

Also, all the other genes are reshuffling at the same time, not just the ones I've singled out. And there are thousands of them. So although I can predict that roughly one in sixteen plants will be yellow sugarsnaps, I can't predict what other traits they may have. They may be tall or short, white flowered or purple-flowered, open-flowered or not-so-open. Any trait from either parent might express itself. And it's also very possible that other unexpected traits may show up that don't belong to either parent, as some genes are interlinked in funny ways and only express themselves in certain combinations. Ultimately I will have to see what I end up with and make my selections from there.

The F2 seeds. You can already see some diversity just in the appearance of the seeds. Some have the grey or tan colour and/or purple speckles inherited from Golden Sweet, others have the green seeds of Sugar Ann. Some are more wrinkled than others, which is an indication of their varying sugar content.

If you've been glazed over during the previous few paragraphs, don't worry. You don't have to understand the science to do these sorts of projects. You can just cross two plants you like the look of and see what happens. See my post on hand pollination for the practical instructions. Or you can wait till I grow mine and post some more photos!


Christina said...

Wow, this is fascinating. I'm really interested in how you get to your final destination. Keep us posted!

Anonymous said...

Rebsie, do you distinguish between sugar snap and mangetout peas? I think of the terms as interchangeable or maybe not? Is the sugar snap a sheller as well as being eaten in the pod?

Rebsie Fairholm said...

Thanks Christina!

John - the distinction is in the thickness of the pods. Pea terminology is a bit ambiguous and I'm not sure if there is a definitive word for these different types! When I talk about mangetout types, I mean the ones with wide flat pods - what Americans call snow peas. Sugarsnaps are the ones that have small fleshy pods and tend to be eaten when the peas have already started to swell a bit. I think when you buy them in supermarkets they're usually called sugarsnaps, but Americans call them snap peas.

Anonymous said...

Oh, now I absolutely want to start breeding my own peas! I'm eager to see how yours turn out.

Rebsie Fairholm said...

Yeah, go for it! It takes a bit of patience but it's rewarding and fun, and even if it doesn't work out you can still eat it!

Steelanger said...

Best is for you to continue selfing the plants until the F7-F8 to have a homogene variety.

Snap pea and mangetout reffers to the same type of peas, there is no difference. Mangetout is the french for eat it whole.
The main difference from the other type of peas is the lack of the fyber in the pods, and a much thicker parchment layer.

Have fun