The Rose and the Avocado

by Laurie Meadows   

August 31, 2019   

Today I learned famous rose breeder Sam McGredy had recently died.

I met Sam when he brought some rose scions into the New Zealand Government MAF office I was working at. He came to arrange the International Phytosanitary Certificate necessary for shipping them overseas. Sam was a genial man, and naturally the conversation turned to his work as a plant breeder.

He was a great storyteller, and he told me a story relating to the nitty-gritty of the life of a working plant breeder. He was working in the heat of his greenhouse, sweat dripping from his brow, engrossed in transferring pollen from the chosen paternal variety onto the stigmas of the maternal parent, when he became aware he had an unexpected visitor.

A polite and very earnest young Japanese man introduced himself as a beginning rose breeder interested in picking up practical tips from the master. Noticing Sam was using a finger to transfer pollen, he asked Sam if he used alcohol to sterilise his finger between crosses. Sam had to explain that he simply wiped his finger clean on his profusely sweating brow!

At one point his operation was generating 60,000 seedlings per year, so speed in physically making the crosses is essential.

The results of his methods speaks for itself.

Sam was instrumental in setting up plant breeders rights in New Zealand, and his floribunda rose ‘Matangi’ was the first plant ever licenced under the new legislation.

According to Sam, some of his creations were so popular that some nurseries overseas illegally propagated them – or understated the numbers they were propagating – so avoiding the breeders rightful royalty.

Sam was familiar with the major nurseries, of course, and he told me he would hire a light aircraft to fly over the nurseries he suspected of cheating on him. It was relatively easy to spot blocks of his cultivars in flower from the air, and assess the size of the stock!

On the same day I read of Sam’s passing, I also learned the avocado genome has been sequenced.

The work was a collaboration between workers in Mexico, Sweden, USA, Canada, Belgium, Singapore, Australia, Spain, Denmark and France.

This achievement is the key to the door to finding and naming avocado genes that express traits of interest, from flower bud initiation to fruit skin color.

The effort to bring this project to reality is impressive. Multiple institutions across countries, complex models, sophisticated software, highly technical biochemical processes.

Why is it so important for avocado breeding?

Because conventional breeding of the sort that Sam McGredy did is all but impossible for avocado. Breeding for multiple traits – moderate tree size, self fertility, good production, high quality taste, no discernible fiber in the flesh, moderate seed size, skin that peels easily, skin that is not too thick and not too thin, fruit that ripens evenly from base to neck, fruit of ‘the right size’, fruit that cool stores well – requires high populations to select seedlings from, and it probably requires multiple unrelated families under selection at the same time.

Worst of all, it is prohibitively difficult to succeed in deliberate hand pollinations – which means difficult to produce seedlings whose parentage is known with certainty.

The bottom line is that very large hectareages need to be locked up in slow-to-flower trees for a long time, even while there is uncertainty of the genetic worth of one parent. And even when there is almost no information on the mode of inheritance of the characters of interest the parent (s) display.

Someone has to provide the land, the running costs, and the highly trained expertise. But not just the expertise, but the experience and insights that can only build up through years of interaction and observation with the test subjects. Given the difficulties in breeding, that’s not an easy situation to sell to anyone!

But now that the genome has been sequenced, it opens the way to use the genetic code for traits of interest that has been identified in other species (from tomatoes to poplar trees) to be compared to the raw code of the avocado genome. If it matches, even partially, some part of the avocado genome, then that section of the avocado genome may be coding for that trait.

Once a match is found and confirmed as being the gene controlling the trait of interest, genetic ‘probes’ based on these snippets of code can be artificially constructed (and presumably patented). These probes can be run on tissue samples from avocado seedlings to see if that particular seedling carries the desired quality. Any that don’t can be thrown out – saving very large amounts of time, space, and money.

The same check can be run over and over on the same seedlings, but using different traits of interest as the screen. For example, in principle, a probe could be developed for every trait of interest mentioned above. Of course, it’s not as simple as that. Someone, sometime, has to find and describe the gene of interest in another species and deposit that information in the communal database. But traits may be controlled by more than one gene. Or other, unknown genes may act as on/off switches to expression of the genes of interest.

All this takes time, effort, and money. And access to money always has competing interests.

For simple single-gene mediated traits it may be simpler to copy that gene from an avocado tree with a gene expressing that trait, and simply paste it into a seedling that is perfect in all respects – except one. Perhaps the seedling has everything the industry wishes for, but has green skin, and the industry considers a color change to indicate ripening is desirable from the consumer viewpoint. If purple skin color was controlled by a single gene (and no other gene interfered with its expression), than a copy (or copies) of the ‘purple skin’ gene could be inserted via gene editing technology.

So this is a significant advance. But it also signifies a shift. A shift of resources to those with the necessary institutional money and technical sophistication. It raises some very interesting questions about cooperation, patents, funder oversight, collaborator rights across international borders, affect of domestic foreign policy on projects (the domain of science diplomacy), and more.

The difference between the methods of Sam McGredy and these methods is obviously very great. But no-one should imagine that, in general, conventional methods have been made obsolete. Very far from it. But in the specific case of avocado, the door to a slow acceleration in progress is creaking open.

But nothing changes the question – what exactly are you hoping to achieve, and what facts and projections have you considered in deciding on your goal?