x Mangave ‘Bloodspot’ going to flower

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Re: x Mangave ‘Bloodspot’ going to flower

Post by Tony R »

Colin Walker wrote: Tue Jun 14, 2022 5:08 pm The generally accepted current concept of Tulista is as a simple split from Haworthia for what was subgenus Robustipedunculares, i.e. the 4/5 big chunky-growing spp. such as T. marginata.

It was a pity Tulista and Haworthiopsis were not accepted in the 2nd edition of the Handbook though?!
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Re: x Mangave ‘Bloodspot’ going to flower

Post by Phil_SK »

Colin Walker wrote: Tue Jun 14, 2022 5:08 pmNobody else follows Gordon with his concept of Tulista
Which is rather the point: it isn't "slight differences in the DNA" that lead to monotypic genera but the choices made in seeking to create monophyletic genera. There's (nearly?) always another option that involves either broadening and reducing genus numbers or shuffling species between genera. It's just that these options tend to be messier or less popular, less followed than the path that leads to lots of small/monotypic genera.
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Re: x Mangave ‘Bloodspot’ going to flower

Post by ralphrmartin »

Colin Walker wrote: Tue Jun 14, 2022 9:29 am Agreed Ralph there's no tightly defined definition of a genus. However, with modern molecular evidence, we're getting closer. So, the components of a genus, species or whatever rank you wish to consider, have to have a common evolutionary origin and form a discrete branch (clade ) in the family tree.
I agree in principle with Colin's idea that whatever we stick a label on should be a discrete branch of the tree, but there are still some sticky issues:

(1) Clades, just being bits of the tree with a common ancestor, are a more generally useful idea than species or genera, which are (or should be) just names for clades which satisfy certain additional requirements (unable to reproduce with others outside the clade, for example). The requirements should in principle be fixed according to the level (species, genus, etc) we are considering. But in practice these additional requirements are not strict - applying commonly used ones about being able to reproduce, we not only get intraspecific hybrids, but intrageneric hybrids. If you tweak them to get "common-sense" answers, they become subjective.
A more objective way of doing it would be to say that things within a certain genetic distance of each other should be considered to be a genus, or a species, for example. However, I doubt that they can be made objective - how do you weight the importance of removing or duplicating a gene, with putting it somewhere else on the chromosome, or a mutation, for example? What about aberrant organisms with extra chromosomes? Furthermore, epigenetics can turn genes on or off, which can muddle things further.

(2) It's not really a tree anyway. (It's a directed acyclic graph, DAG, if you want to be precise - directed just says it is clear which is parent and which is offspring, and acyclic just means you can't be your own ancestor). We call it hybridisation if two reproducing organisms' last common ancestor was a long way back up the "tree". Assuming it's a tree is a simplification that is typically used in computer based analysis of DNA, to make the mathematical problem of determining the most plausible explanation easier. Other assumptions are made too, e.g. that the simplest explanation is correct, so for example an explanation in which something is always present is preferred over a less likely answer in which it is lost and regained. Less likely of course does not mean mean "never happens". How often hybridisation is important for cactus evolution, i don't know, although some seem to hybridise pretty readily.

Anyway, the useful side of DNA analysis is that it can show us mistakes, where big differences in DNA between things we though were closely related are actually far apart (because they looked similar, for example). But I can't see how it can really be used to resolve or justify questions of what a species is, or a genus is, in terms of a chunk of the DAG. And it can only give us the most likely answer, given some assumptions, which is not the same as what actually, historically happened. When there are so many "species" in the world, less likely explanations will be the real ones from time to time.
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Re: x Mangave ‘Bloodspot’ going to flower

Post by Colin Walker »

ralphrmartin wrote: Tue Jun 14, 2022 8:05 pm
Colin Walker wrote: Tue Jun 14, 2022 9:29 am Agreed Ralph there's no tightly defined definition of a genus. However, with modern molecular evidence, we're getting closer. So, the components of a genus, species or whatever rank you wish to consider, have to have a common evolutionary origin and form a discrete branch (clade ) in the family tree.
I agree in principle with Colin's idea that whatever we stick a label on should be a discrete branch of the tree, but there are still some sticky issues:

(1) Clades, just being bits of the tree with a common ancestor, are a more generally useful idea than species or genera, which are (or should be) just names for clades which satisfy certain additional requirements (unable to reproduce with others outside the clade, for example). The requirements should in principle be fixed according to the level (species, genus, etc) we are considering. But in practice these additional requirements are not strict - applying commonly used ones about being able to reproduce, we not only get intraspecific hybrids, but intrageneric hybrids. If you tweak them to get "common-sense" answers, they become subjective.
A more objective way of doing it would be to say that things within a certain genetic distance of each other should be considered to be a genus, or a species, for example. However, I doubt that they can be made objective - how do you weight the importance of removing or duplicating a gene, with putting it somewhere else on the chromosome, or a mutation, for example? What about aberrant organisms with extra chromosomes? Furthermore, epigenetics can turn genes on or off, which can muddle things further.

(2) It's not really a tree anyway. (It's a directed acyclic graph, DAG, if you want to be precise - directed just says it is clear which is parent and which is offspring, and acyclic just means you can't be your own ancestor). We call it hybridisation if two reproducing organisms' last common ancestor was a long way back up the "tree". Assuming it's a tree is a simplification that is typically used in computer based analysis of DNA, to make the mathematical problem of determining the most plausible explanation easier. Other assumptions are made too, e.g. that the simplest explanation is correct, so for example an explanation in which something is always present is preferred over a less likely answer in which it is lost and regained. Less likely of course does not mean mean "never happens". How often hybridisation is important for cactus evolution, i don't know, although some seem to hybridise pretty readily.

Anyway, the useful side of DNA analysis is that it can show us mistakes, where big differences in DNA between things we though were closely related are actually far apart (because they looked similar, for example). But I can't see how it can really be used to resolve or justify questions of what a species is, or a genus is, in terms of a chunk of the DAG. And it can only give us the most likely answer, given some assumptions, which is not the same as what actually, historically happened. When there are so many "species" in the world, less likely explanations will be the real ones from time to time.
Wow Ralph, that's some essay. :grin: :grin: :grin:
My quick fire earlier response was just that, not an expanded assessment of current taxonomic thinking and procedure based on molecular evidence.

I don't actually disagree with anything you've said. The main point I was making is that molecular evidence provides a more objective basis for making taxonomic judgements. Subjectivity is still part of the mix for sure, but as we move forward this will play less and less of a role.

What you've not covered of course are big issues about DNA sequencing being high selective and current comparisons are based on only small fractions of total plant genomes. At some point many years from now we will be comparing whole genomes. :grin:

As current background on this, the Sanger Centre/Wellcome have just announced that they're planning to fully sequence most of the British flora along the lines of the Human Genome Project. Once the whole world's flora has been fully sequenced the taxonomic landscape will look very different to what it is today.

However, plant genomes are generally large compared to those of animals, whilst bacterial and viral genomes (e.g. Covid/Coronavirus) are tiny by comparison. Consequently total sequencing of every plant species on the planet is a mega project.
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Re: x Mangave ‘Bloodspot’ going to flower

Post by Colin Walker »

For those of you into mangaves, the latest Amer. J. has just arrived here and there's a very good article on the history and development of these hybrids:

Tony Avent. Magnificent mangaves. CSJ(US) 94: 82-87, 2022.

Coincidentally I've just chopped up my large clump of 'Macho Mocha'. This had got very tatty, being a haven for marauding molluscs. :shock:
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Re: x Mangave ‘Bloodspot’ going to flower

Post by Diane »

Update!

After a few weeks of nothing to report, things have started moving quickly!

Latest photo:
X Mangave ‘Bloodspot’
X Mangave ‘Bloodspot’
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Re: x Mangave ‘Bloodspot’ going to flower

Post by Ernie »

(tu)
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Re: x Mangave ‘Bloodspot’ going to flower

Post by Diane »

A rather belated update: flower spike is now 27”/69cms tall. It really took off last week, when the weather warmed up.
X Mangave’Bloodspot’
X Mangave’Bloodspot’
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Re: x Mangave ‘Bloodspot’ going to flower

Post by edds »

They look almost unreal. Thanks for sharing.
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Re: x Mangave ‘Bloodspot’ going to flower

Post by juster »

Goodness that's really grown fast, keep us posted Diane.
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