Seed Quality - Inbred Cacti?

[StevenT]

I don't know much (?anything) about this subject, but is there a question to be asked about the parentage of the c&s seed that is often sown by growers?
It seems to me that there is a lot of open pollinated seed around which is simply labelled with the name of the seed parent it has been gathered from. Given that most amateur collections comprise a lot of 'species' but often only one plant (or a few at most) of each one, does this mean that such seed is probably from unrecorded hybridisation?

So is 'hybrid vigour' occurring and is this a factor?

Steven

[Apicra]

It is good that you are taking the trouble to prefer c&s with habitat data. I think nearly all growers making seed for distribution would take precautions to isolate the plant from unintended cross-pollination and grow multiple clones of the target. The danger of hybridisation is widely-known and real.

It is unfortunate that in botanical-speak, the opposite phenomenon to "inbreeding depression" appears to be called "hybrid vigour". The word "hybrid" implies crossing between two species to most people. But of course this phenomenon can occur between any two plants that are genetically distinct, even between plants classified as the same species, subsp. or whatever. The wider the genetic difference, the more likely it is of course that there will be some barrier to cross-pollination, but that is another large and complex topic. Perhaps "out-breeding vigour" would be more appropriate for this discussion.

Best wishes,
DT

[StevenT]

I very much hope you are right Derek about growers who produce seed for distribution taking precautions against hybridisation. I just wish that this was stated in seed lists. Unless I missed it, the recent BCSS one for example, had no reference to the seed having been produced from controlled pollination.

Steven

[Apicra]

Something vendors really ought to add to seed packets is the month+year of harvest, as I've complained before. This would give confidence that we are not buying long out-of-date seed.

Best wishes,
DT

[Apicra]

I've always found botany to be a complex and "slippery" subject. Everything appears to be variable and relative. This is unlike physics, where beautifully simple laws apply (just about) universally. In botany, concepts often have many exceptions, or there are other ways of achieving the same result. And what is essentially the same process is called different things in different contexts. But c&s evolution is worth discussing because it might inform our cultivation techniques and certainly impacts upon taxonomy.

However, in the real physical world, the only thing that matters to a plant is survival through time. It does not care about its historic evolutionary path, breeding mechanism or if it is a species, hybrid or whatever - these are just human constructs used to enable people to talk about plants.

This grand introduction is building up to a possible explanation about c&s inbreeding. But first, more detail from that maize experiment (Jones 1924):

Parents Crossed Generations
P1 P2 F1 F2 F3 F4 F5 F6 F7 F8
No. generations selfed 17 16 0 1 2 3 4 5 6 7
Mean height 67.9 58.3 94.6 82.0 77.6 76.8 67.4 63.1 59.6 58.8
Mean yield 19.5 19.6 101.2 69.1 42.7 44.1 22.5 27.3 24.5 27.2
(Oh dear, tables don't work very well!)

I'm only halfway through reading the textbook, but a credible explanation for the lack of inbreeding depression in c&s is (pretty obviously really) that many could have been repeatedly inbred already. Thus further inbreeding in cultivation does not reveal much change, as shown above by the later inbred generations of maize. Many populations of c&s are small and localised, so when a disruption occurs e.g. prolonged drought, quantities and genetic diversity are reduced by inbreeding. This is apparently called a genetic diversity "bottleneck" and is essentially the same process as when new populations are founded by very few individuals via dispersal, when it is called a "founding effect". Perhaps this evolutionary process is what IanW was describing earlier, in which case I apologise for misunderstanding, but the comments did not make sense to me in the context of the maize experiment.

So how can c&s survive in habitat if they "lack vigour"? It sounds bad, but perhaps they just avoid competition such that it does not matter. C&s usually grow where potential competitors cannot i.e. dry places, where water-storage gives an advantage. And after all, we already know that most get by with the "major handicap" of inefficient CAM, when competing against more efficient C3/C4 photosynthesis.

So to put it another way, many plants need "outbreeding vigour" to compete, like the F1 generation above, but many c&s appear to exist in isolated niches with little competition and are like the highly inbred parental lines above. Or one could just say more simply that inbreeding depression does not apply to many c&s!

These comments do not apply just to small c&s like Mammillaria pectinifera, although that type is what comes to mind. It could just as easily be a large branching tree-like cactus that has small inbred populations, lacking gene diversity.

It should be emphasised that all this about inbreeding is not proven science, but just informed speculation. Appropriate experiments could be devised to test these ideas but is there circumstantial evidence already?

Best wishes,
DT

[StevenT]

Are there no more recent experiments than that of 1924? Nigh on a century seems a very long time for there not to have been more experiments to investigate inbreeding...

Yes, c&s often don't compete with C3/C4 plants. For example Melocacti grow on 'islands' of stony ground in regions of lush vegetation. So for c&s, perhaps it's a case of 'life in the slow lane' being actually better for survival in harsh environments where vigorous growth would be a disadvantage?

Are c&s populations inbred? The Mexican tree cacti for example that are pollinated by bats that fly over large distances. Hummingbirds and sunbirds also cover big distances.

Steven

[Apicra]

Hi Steven,

I expect there are more recent studies about inbreeding than 1924 - it is just standard practice for a text book to reference the earliest study that clearly demonstrates a process. It would indeed be good to know more about how many plant groups have been found to be impacted by inbreeding depression?

Yes, Melocacti do appear to be a good example of cacti avoiding competition.

I only wanted to say that many c&s are probably naturally inbred, not that all c&s are. Population size is indeed influenced by pollinator range and one would expect bird or bat pollinated species to have widespread & numerically large populations. But even tall, bat pollinated, tree-like cactus populations must reduce towards extinction sometimes ... and then they may be vulnerable to inbreeding depression if they usually "enjoy" outbreeding vigour.

Best wishes,
DT

[Jim_Mercer]

A little Google "research" found this page http://scientificgardener.blogspot.co.u ... ssion.html which seems to suggest that the maize trials quoted in the original post may not be relevant to our plants as they are "outbreeders" so require large populations to avoid inbreeding depression whereas many of the plants of interest to us are I think "inbreeders" so need a smaller population.

[Apicra]

The plants that are call "inbreeders" in that article would be called self-fertile elsewhere, I believe. Few cacti follow that path e.g. Fraileas. They do have different biology and obviously survive with extreme inbreeding. The vast majority of cacti are "outbreeders".

It is interesting that this article about growing food crops quotes a figure for the number of plants needed to avoid inbreeding depression when crossing. My text book suggested 50, this article suggests 100 for usual outbreeders and only 20 for self-fertile species. It is very unlikely any of our c&s seed producers has space to cultivate that many, confirming that in cultivation, we are unwittingly trying to growing seeds at risk of inbreeding depression.

Best wishes,
DT

[esp]

I wonder if the issue is a bit less critical (at least from a practical point of view) for many C&S, in that the generational time and mother plant longevity will be longer (a few years to many years) rather than a single year for the annual grain and vegetable crops discussed in the studies.

I assume many C&S seed producers in cultivation tend to have a small number of large, old mother plants for seed production, which will produce over many years, rather than having a number of smaller, repeatedly interbred, young mother plants as the seed producers. So whilst a lot of seed may be produced from a small selection of parents, at least many of these parents will not be multiply interbred across a small cultivated gene pool over many generations.

Neglecting where gene pools are refreshed by seed collected direct from habitat, I guess this problem will be least pronounced in plants that take many years to reach maturity (say, Carnegia, Ferocactus or Denmoza), moderately pronounced in smaller plants (say Parodias, Escobarias and many, many C& S that typically take a few years to mature), and most pronounced in highly interbred, quickly maturing seed lines (e.g Lithops cultivars, or new dwarf introuductions, like novel Turbincarpus species which will be grafted as seedlings, and highly interbred for a few generations at a very young age to produce a population of seed or plants for sale).

Of course, vegetatively reproduction will not contribute to interbreeding, for species propagated in this way, although it will lead to distribution of a limited range of genetic material. So clustering Mammillarias or Rebutias could potentially be candidates for interbreeding problems, but at times they may be intensively reproduced vegetately, so on average may be derived from fewer generations of interbreeding than with their non-clustering relatives which will only be seed grow.