Friday, 25 June 2010

Great Ocaspectations (2) Oca Genetics: This Much I know

OK - it's time to disperse the cloud of unknowing that obscures the summit of Mt Oca.

I'm going back to basics, just like former Prime Minister John Major did; I hope it works out better for me than it did for him. I make no apologies for using the technical terms for various processes - if I've got to learn them, so have you. "This Much I Know" could be rephrased more truthfully as "My Brain Hurts".

Brace yourselves.

Oca's genetic code is located on eight different chromosomes. In a "normal" diploid species they occur as pairs, which means 2 x 8 =16 chromosomes in total, see below:

In oca's case, however, the picture is more complicated: it is is a polyploid, with four sets of each chromosome pair, (2n =8x = 64). This means it is an ocatoploid, sorry, octoploid, with 64 chromosomes in total:

It is also an alloploid, formed when the different genomes of precursor species joined together. The latest evidence suggests that it may be an auto-alloploid. Resist the lure of a darkened room and bear with me for an explanation: this is gonna hurt me more than it hurts you.

What apparently happened is that one diploid parental species with the diploid chromosomal complement AA, doubled its chromosomes to form an autotetraploid AAAA (2n= 4x = 32, no chromosomes from any other species):

This then combined with two separate diploid species (BB and CC) or one allotetraploid (BBCC), itself formed by the uniting of two diploid species and nothing to do with the BBC, except perhaps in that they both contain a lot of repeats.
The result is AAAABBCC, the auto-alloploid Oxalis tuberosa we all know and love, with half the genome coming from species A, the other half from the B and C genomes, one quarter each.

The identity of these species is still not entirely clear, although likely suspects are Oxalis chicligastensis from northwestern Argentina and Bol/WT, an as-yet-unnamed Oxalis species from Bolivia.

Much of this information comes courtesy of Our Lady of the Ocas, Dr Eve Emshwiller, whose pioneering work on the genetics of Oxalis tuberosa is starting to unravel the mysteries of this crop. Her latest paper from 2009 gives much more detail on this and also has a lovely picture of a collection of oca tubers that are positively indecent in their diversity. Food porn I've heard of, but tuber porn (which this surely is), is a new and welcome addition to my life. Not such an inappropriate connection as it happens: oca has strong sexual associations in Quechua language and culture. The phrase oqa-tarpu, for instance, which literally means "planting ocas", doubles as a term for sexual intercourse. Another useful piece of information is that an oqaratu is a stupid, lazy or vacillating person. Feel free to spice up your domestic disputes about the gone-off milk, the missing car keys or the unpaid bills with this useful term of abuse.

In terms of character inheritance and plant breeding, where does this leave us? We know that oca is an obligate outcrosser (allogamous) with a system of self incompatibility based around three stylar morphs, short medium and long (tristyly). The genetics of this have been elucidated here. Basically, the inheritance of flower morphs was shown to be tetrasomic (involving four versions of the same chromosome at meiosis) and diallelic with two alleles of each of two genes: S,s and M,m, with S preventing the expression of the M gene - a process known as epsistasis. Various combinations of the above M and S genes and their alleles give rise to the three flower morphs. And it's all happening on the autotetraploid subsection of the genome.

Don't confuse epistasis with dominance. Dominance is caused by the dominant allele of a gene overriding a recessive one. Epistasis involves separate genes in which one has a cancelling effect "upstream" of the others - like trying to change channels on the telly when it isn't plugged in yet. Or (how's this for topicality?) head straight to Daughter of the Soil's post here for a practical demonstration.

So if all the flower morph activity (a big if) is located on the autotetraploid AAAA chromosome chunk, there must be a whole lot more going on over at the BBCC section of the genome. There will be plenty of genes located in these other ancestral genomes which may not be inherited tetrasomically. Which genes are where and which ones are inherited in which way are as yet, to me anyway, completely and utterly unclear. I feel the mists descending and the summit of Mt Oca disappearing from view. That or a migraine coming on.

It's probably a safe bet however, that oca is heterozygous for many traits and that these will segregate in subsequent generations in all sorts of complicated ways due to its mixed up genome. Oh joy. Mendel was lucky he chose peas. Nevertheless, I would imagine that the traits will fall into two categories, qualitative and quantitative.

Qualitative traits are those likely to be under the control of single genes or at least only a few genes: a binary on/off, present/absent, as shown in classic Mendelian inheritance (see Daughter of The Soil's blog for a thorough and lucid exposition of all of this).

Quantitative traits are ones which grade seamlessly from one to another along a continuum. They are under polygenic control, that's to say many genes, perhaps on different chromosomes, have an influence on any particular characteristic. These include, in some other tuber crops at least, traits such as yield, tuber shape and flesh quality.

I suspect, although I don't know, that tuber colour and plant form in general are maternally inherited - I should be able to put this to the test when I harvest the tubers in the autumn. Among the more obvious traits I can detect above ground are stem colour (red/green) underside of leaf (purplish/green):

And the absence or presence of axillary colouration. It would be fun to try and work out the mechanism of inheritance in these cases. If only I knew who their parents were.

Under the circumstances of my overwhelming ignorance, it is probably as well to follow the time-honoured route used by some potato and sweetpotato breeders - recurrent mass selection. It's a case of crossing varieties in every possible combination, then sowing and selecting their progeny, eliminating the feeble or substandard and keeping the best for the next breeding cycle. I could also do a little controlled crossing here and there and see what happens.

My main concern is that my seedlings have been derived from a handful of varieties, possibly less. Ideally you would start with a population of at least twenty unrelated individuals in the "crossing block". All is not lost however, as it should be possible to incorporate genes from new varieties by crossing them with the other ones to introduce more genetic variability - a process known as introgression.

There you go - that's close to the sum total of my knowledge of oca genetics at present: not a lot. That darkened room awaits me. What about you?

Saturday, 19 June 2010

A Surprise Ocaurrence - What's That Lurking in My Lathyrus?

Passing one of my caremyle pots the other day, I did a sudden double take. There, nestled beneath the foliage, were two little seedlings, with trifoliate leaves.

Intrigued, I looked closer. Not clover, nor Oxalis corniculata, but little oca plants - definitely. Unlike Queen Victoria, I was amused.

I must have repotted the caremyle with some spent compost containing oca seeds, which had then decided to germinate. I quickly extricated them and planted them in modules; I'll reunite these foundlings with their extended family in the oca plot when they reach the age of majority. Scrutinising the surface of the pot more closely , I noticed further seedlings starting to show.

Not long afterwards I was hoeing off some weeds and oca volunteers which I wanted to remove from a bed. I noticed that one of the ocas, small and stocky and moments before a picture of health, actually had cotyledons. This was none other than my first oca seedling to appear spontaneously outside in one of our beds and I had, in my enthusiastic ignorance, decapitated it. I held the the bisected beauty in my hands with an expression of mute incomprehension. No I didn't - I swore with undignified intensity and rapidity. Regaining my composure quickly (all of 30 minutes), I replanted the rootless stem and I'm hoping that I can persuade it to re-root. It at least proves that oca seeds can germinate and develop outside satisfactorily, although how large a plant it would have made before the winter is anyone's guess, especially with me acting plum loco parentis. If it survives, I'll name it Lucky, after that darn dog from that ol' poster:

Friday, 11 June 2010

Great Ocaspectations (1) Overdosing on Oca

Here's a view of my oca seedling bed as it looked a couple of weeks ago. Each pot contains a single seedling raised by yours truly this year, from the seeds I harvested last autumn. It may or may not be an idle boast, but I reckon that I've now got the largest collection of oca germplasm in the whole county of Cornwall. It all looks uncharacteristically neat and tidy, which rather belies the frenetic panic with which they were planted a few weeks ago.

Some friends had invited us over for tea in their lovely moorland garden. It was a glorious afternoon, with the hot and penetrating sunshine moderated by a pleasant breeze and an almost alpine aspect. I was aware that afternoon was passing into evening, but the sandwiches, cakes, drinks and good conversation made the thought of lifting my corporeal bulk from the lounger even more unappealing. The sun's rays were weakening - and so was my enthusiasm for oca planting exploits. Oh well, tomorrow perhaps? I was gently but firmly reminded of my promise to get the job done. Suitably chastened, I hurried home to collect seedlings, pots and other equipment and get to work.

Working feverishly to plant and label them all before the remaining light faded, I was reminded of Kirosawa's film Dersu Uzala. In one memorable scene, the Nanai hunter saves the life of the Russian army officer Arsenyev by rapidly building a reed hut to protect them when they are caught out by an unexpected twilight blizzard. If I'd stopped daydreaming and searching for poetic comparisons, I might have got it finished a bit sooner. Thanks to the diminishing light levels I can be fairly certain that my attempts to distribute the seedlings randomly have been successful - if more by accident than design.

By my calculation there were approximately 80 oca seedlings to be housed - the products of semi-controlled crosses - that's to say I knew who their mothers were, even if their dads were strangers to me. I attempted to grow ten seedlings or more of each variety, but some failed to come up or were scythed from below by damping off fungi. That's life. Each seedling from a known mother bears her appellation, RX0909 for instance and then an additional number to distinguish the siblings from one another - 1,2,3 etc.

Ah, but I'd reckoned without another 40 seedlings whose parentage was entirely unknown and I had sown during a what-the-hell moment back in the early spring. Suddenly there they were, pleading with me for a stay of execution - till the autumn at least. There was only one thing for it - I'd have to fit them in somewhere, somehow. No wonder I was working at such a breakneck pace. These seedlings I gave designation RX1001 , 02 and so on. I planted them nearby, turfing out, literally, some other less worthy candidates, a bunch of chillies. What is abundantly clear is that this relationship is entirely one-sided - I am at the beck and call of the ocas. I am their dutiful slave. They've domesticated me.

The canes are there to help prevent the plants from sprawling over and through one another and will also allow me to access the flowers and pods more easily - I hope. The pots will, as they did last year, keep the tubers of each variety separate. Under ideal circumstances I would have been able to give each seedling plentiful root run and space from its neighbours and planted them directly in the ground. Unfortunately, that luxury seems to have eluded me yet again. Anyone who has tried determining the provenance of stray oca tubers scattered through a Somme-like plot will know why I have opted for this horticultural apartheid. I have earthed up the stems once so far and will do so again, in the hope that this will encourage the development of tuber-bearing stolons, just like with potatoes. The bare soil, by the way, will be mulched in due course.

What with these seedlings, the tubers from last year 's seedlings, the wrinkled old retainers and new arrivals courtesy of Frank van Keirsbilck, I must have planted around 140 genetically unique individuals this season. Frank managed to get good germination from the seeds produced by my plants of "Pink Dragon", the variety he raised a couple of years ago. So that adds another 90 or so individuals to this newly evolving secondary centre of oca diversity in NW Europe. I hope we're getting up to the kind of numbers where we might start to see some interesting new traits appearing. We're also reaching the point at which some serious winnowing of duds needs to occur.

But before I get too trigger happy, it is probably as well to pause and consider what combinations of which traits might actually be desirable in those oca seedlings that escape the indignity of being untimely ripped from their pots. Here goes:

1) Early tuber formation during long days/ high yield (large tubers)
2) Profuse and early flowering
3) Vigorous and disease resistant
4) Good tuber quality and flavour

1) The main obstacle to oca's adoption as a food crop at our latitudes is its inability to form tubers during the long days of summer. A handful of marbles harvested in October can only lead to unfavourable comparisons with the potato and contribute to oca's notoriety as an "experimental crop". No, this just won't do: I'm looking for day neutral forms, or ones that can tuberise when days are significantly longer than 12 hours. I won't rest until I find one, preferably several. Perhaps I ought to set a harvest date some weeks before the end of September, lift the plants and keep those bearing the largest tubers.

2) At this stage in the game it is vital to obtain precociously floriferous plants with the ability to set a plentiful crop of pods. Seeds contain novel and unique combinations of genetic material and are the raw material upon which selection, natural or otherwise, works. Can't have too many seeds.

3) Vigour and disease resistance are also very important. Differences in vigour start to show early on in a seedling's life. I suspect vigour could be linked to early flowering and - maybe - early tuber formation. If some seedlings are small and puny and prone to damping off, chances are their offspring will inherit this weakness. I'll weed them out, so that their contribution to the next generation is nil. I'll be watching disease and pest susceptibility too.

4) At the moment my main concern is to get plants that reproduce efficiently, both sexually and vegetatively. That said, I ought to check that the tubers resulting from my mini breeding programme are of acceptable quality and are not too bitter, fibrous or unpleasant in any other way.

In my next post I will discuss what little I know about oca genetics and how I might use this knowledge in my breeding programme.

Saturday, 5 June 2010

It's a Seed, But Not As We Know It, Jim

Actually Jim, it's an anthocarp, a mauka anthocarp. On one of my mauka roja plants. I'll put the tricorder down for a minute and explain. An anthocarp is a specialised fruit found in the family Nyctaginaceae to which mauka (Mirabilis expansa) belongs. And before you ask, yes, I do know what a nail brush is.

Seems like my clumsy fumblings have resulted in a single, successful pollination. Alternatively, it may have happened spontaneously, without any connection to the prodding and poking which I doled out to all the hapless flowers I encountered.

What looked like pink petals in my previous picture weren't - they were colourful sepals standing in for the petals, which went AWOL sometime in the evolutionary history of this family. Similarly, what looks like a ripening seed is, from the botanist/pedant's viewpoint, a fruit, albeit somewhat unlike the common conception of one. It's an achene (dry fruit) wrapped in a persistent calyx - known in botanical shorthand as an anthocarp. In mauka's case this outer wrapping is covered in sticky hairs. These must help the ripe seeds stick to any passing birds or animals which then distribute them to new areas.

Here's a close-up of the achene, divested of its glandular, sticky coat. There are definite ridges present on its surface - a common feature in Mirabilis species.

So, although I don't know whether this mauka propagule is viable, it seems likely that, under some circumstances at least, mauka can self pollinate. This can only be a good thing.
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