Hybridization: Ploidy
If you're arrived on this page and haven't checked out the previous posts full of background information, you may want to do that first and come back here when you're done!
Phylogeny: Who's related to who? (Echeveria vs. Sedum)
Hybrids: Cross-breeding (Mendelian genetics, how-to, and flower basics)
Variegation Genetics: Follows a recessive Mendelian pattern (endosymbiotic theory)
Hybridization: Ploidy: YOU ARE HERE
Echeverias can easily be cross-bred with several other succulents in genera like Pachyphytum, Graptopetalum, and Sedum (Uhl, 1982a, 1982b). This is probably because they are all closely related, despite the differences in naming. You can read more about how taxonomic binomial names don't always accurately reflect DNA sequencing here.
Categories of Fertile Hybrids
Hybrids are generally infertile, just like how a horse + donkey = infertile mule. However, there are some hybrids that are fertile that fall within two categories: 1) the parents are closely related and have the same chromosome number 2) the parents are polyploidy
Biology Terms
First, let's walk through some biology terms. I suggest going to Khan Academy if you need a refresher on meiosis, but that isn't necessary to understand this post. (I'm a chemistry person, so I'm literally googling this now to make sure I tell you accurate information. It's been a while since I've had cell biology!) At the beginning...DNA!
What is DNA? It stands for deoxyribonucleic acid and it is the main constituent of chromosomes that contains genetic material. A cell's set of DNA is referred to as its genome. DNA is made of a sugar phosphate backbone (the sides of the ladder) and the rungs of the ladder are the nitrogenous bases: Adenine, Thymine, Guanine, and Cytosine.
The DNA is then twisted up in a double-helix formation. The shape of DNA was discovered by Rosalind Franklin using x-ray crystallography to obtain Photo 51. (This is a technique that my bioinorganic lab uses to "see" the structure of proteins).
The double helix is then wrapped around proteins called histones similar to yarn around a spool. This is the chromatin. Chromatin, chromotids, and chromosomes all refer to DNA in different forms/shapes. Chromatin is just like loose spaghetti made of DNA hanging out in the nucleus. During cell division (mitosis for body/somatic cells and meiosis for sex cells), the DNA/chromatin replicates before condensing into the familiar chromosome X-shape.
If I were to "cut" the X of the chromosome in half vertically, each half would be called a sister chromatid.
So now, what about chromosome number and ploidy?
Chromosome number means the number of chromosomes in that organism. For instance, for humans the chromosome number is 46. These 46 chromosomes are arranged in two difference sets: one set from mom, one set from dad. A pair made of the chromosome from mom and the matching one from dad is referred to as a homologous pair, and each chromosome in the pair is a homologue of the other (with the exception of chromosome pair #23, the X and Y chromosomes). Because these chromosomes are arranged in these two sets, humans are said to be diploid. Ploidy refers to the number of sets of chromosomes. In diploid organisms, we say they have 2n chromosomes. So for humans, we know there are 46 chromosomes and that we are diploid, thus following the 2n pattern. This means that humans have n = 23. Dysploidy means that there is variation in the chromosome number within a given population that is less than a whole set of chromosomes.
What about succulents?
Succulents are...difficult. Because succulents have evolved all over the world, their DNA is a little funky at best. Echeverias in particular also grow in isolated places in small groups, like on cliffs or mountains, which will cause them to rapidly evolve different genomes. Studies have looked at cacti and succulents and have found a surprising amount of variation within the same species, from diploid to polyploid and some things in between due to dysploidy. For instance, Echeveria lutea has n=12 while Graptopetalum macdougallii has n = 92. Here are some more examples:
Echeveria peacockii has n = 15;
Echeveria lilacina n = 27 and Echeveria colorata n = 27;
Pachyphytum oviferum (moonstones) n =33;
Graptopetalum amethystinum (lavendar pebbles) n = 34;
Graptopetalum paraguayense n =68;
Echeveria elegans (historically, Echeveria potosina and one of the parents of Perle von Nurnberg) n =112.
Clearly, there is a lot of variation, but does this really matter? One succulent, Pachyphytum hookeri which has n = 32 was able to be cross-bred with 125 other succulents across eight different genera. (This includes around 80 species of Echeveria.) It almost seems as if, practically speaking, this whole chromosome business doesn't really matter. (Reference 2)
How nature basically takes care of this ploidy problem for the average succulent breeder
They found that when two succulents with very different chromosome numbers came together to make a hybrid, either autosyndensis or allosyndesis occured. Autosyndesis is when chromosomes from both parents paired up because they were homologues, in other words, it creates a hybrid where some of its chromosomes that are paired came from the same parent. Allosyndesis is when they pair but they are not homologues, or is a hybrid from two different parents.
Autosyndesis
So, let's say you've got a diploid called S. allantoides (58 chromosomes) and then you've got a tetraploid called E. secunda (32 chromosomes) and they make a triploid hybrid. How does this happen? Well, the S. allantoides forms 29 pairs of chromosomes and the E. secunda forms 16 pairs of chromosomes. This matches the observed 45 chromosomes, indicating that autosyndesis is strongly preferential, where the chromosomes all match up with each other from the same parent. The hybrid has very fertile pollen. This example comes from reference 3.
Allosyndensis
If you had two diploid species, then during meiosis, you would only have one chromosome of each kind that could not pair with each other from the same parent. It would be forced to pair with the corresponding set from the second parent, resulting in allosyndensis. It is noted that there are never any more chromosome pairs observed than the number of the least-numbered chromosome parent.
What does this mean for me?
This all adds up to...succulents can take care of themselves. Even when faced with a partner of different n-values, they are able to sort themselves out. Hopefully, this post gave you some more insight into why it is possible for you to go cross-breeding all over the place, sashaying across genera boundaries and mixing up species for your own pleasure. It should also help you understand the complications of genetics and why, most often, you end up with a hybrid that is infertile (can't make seeds, but can propagate by leaf pullings). It all comes back to chromosomes.
I hope you enjoyed this week's Succulent Science Sunday! Please hit SUBSCRIBE to keep up-to-date on more succulent science!
References:
1) http://www.crassulaceae.com/crassulaceae.com/botanik/pflanzen/botanzeige_scan_en.asp?gnr=1610&scan=122880-2&cat=7&name=Echeveria
2) The Problem of Ploidy in Echeveria (Crassulaceae) I. Diploidy in E. Ciliata
Author(s): Charles H. Uhl
Source: American Journal of Botany, Vol. 69, No. 5 (May - Jun., 1982), pp. 843-854
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2442977
Accessed: 08-07-2018 03:52 UTC
3) Polyploidy, Dysploidy, and Chromosome Pairing in Echeveria (Crassulaceae) and Its Hybrids
Author(s): Charles H. Uhl
Source: American Journal of Botany, Vol. 79, No. 5 (May, 1992), pp. 556-566
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2444868
Accessed: 08-07-2018 03:52 UTC
Phylogeny: Who's related to who? (Echeveria vs. Sedum)
Hybrids: Cross-breeding (Mendelian genetics, how-to, and flower basics)
Variegation Genetics: Follows a recessive Mendelian pattern (endosymbiotic theory)
Hybridization: Ploidy: YOU ARE HERE
Categories of Fertile Hybrids
Hybrids are generally infertile, just like how a horse + donkey = infertile mule. However, there are some hybrids that are fertile that fall within two categories: 1) the parents are closely related and have the same chromosome number 2) the parents are polyploidy
Biology Terms
First, let's walk through some biology terms. I suggest going to Khan Academy if you need a refresher on meiosis, but that isn't necessary to understand this post. (I'm a chemistry person, so I'm literally googling this now to make sure I tell you accurate information. It's been a while since I've had cell biology!) At the beginning...DNA!
What is DNA? It stands for deoxyribonucleic acid and it is the main constituent of chromosomes that contains genetic material. A cell's set of DNA is referred to as its genome. DNA is made of a sugar phosphate backbone (the sides of the ladder) and the rungs of the ladder are the nitrogenous bases: Adenine, Thymine, Guanine, and Cytosine.
 |
| Photo via wikimedia |
The DNA is then twisted up in a double-helix formation. The shape of DNA was discovered by Rosalind Franklin using x-ray crystallography to obtain Photo 51. (This is a technique that my bioinorganic lab uses to "see" the structure of proteins).
The double helix is then wrapped around proteins called histones similar to yarn around a spool. This is the chromatin. Chromatin, chromotids, and chromosomes all refer to DNA in different forms/shapes. Chromatin is just like loose spaghetti made of DNA hanging out in the nucleus. During cell division (mitosis for body/somatic cells and meiosis for sex cells), the DNA/chromatin replicates before condensing into the familiar chromosome X-shape.
 |
| Left to right: DNA ladder, into a double helix, wrapped around histones, condensed into X-shaped chromosomes found int he nucleus of cells |
If I were to "cut" the X of the chromosome in half vertically, each half would be called a sister chromatid.
 |
| Like my beautiful artistic skills? Above, a chromosome I drew using MS Paint. |
So now, what about chromosome number and ploidy?
Chromosome number means the number of chromosomes in that organism. For instance, for humans the chromosome number is 46. These 46 chromosomes are arranged in two difference sets: one set from mom, one set from dad. A pair made of the chromosome from mom and the matching one from dad is referred to as a homologous pair, and each chromosome in the pair is a homologue of the other (with the exception of chromosome pair #23, the X and Y chromosomes). Because these chromosomes are arranged in these two sets, humans are said to be diploid. Ploidy refers to the number of sets of chromosomes. In diploid organisms, we say they have 2n chromosomes. So for humans, we know there are 46 chromosomes and that we are diploid, thus following the 2n pattern. This means that humans have n = 23. Dysploidy means that there is variation in the chromosome number within a given population that is less than a whole set of chromosomes.
Succulents are...difficult. Because succulents have evolved all over the world, their DNA is a little funky at best. Echeverias in particular also grow in isolated places in small groups, like on cliffs or mountains, which will cause them to rapidly evolve different genomes. Studies have looked at cacti and succulents and have found a surprising amount of variation within the same species, from diploid to polyploid and some things in between due to dysploidy. For instance, Echeveria lutea has n=12 while Graptopetalum macdougallii has n = 92. Here are some more examples:
Echeveria peacockii has n = 15;
Echeveria lilacina n = 27 and Echeveria colorata n = 27;
Pachyphytum oviferum (moonstones) n =33;
Graptopetalum amethystinum (lavendar pebbles) n = 34;
Graptopetalum paraguayense n =68;
Echeveria elegans (historically, Echeveria potosina and one of the parents of Perle von Nurnberg) n =112.
Clearly, there is a lot of variation, but does this really matter? One succulent, Pachyphytum hookeri which has n = 32 was able to be cross-bred with 125 other succulents across eight different genera. (This includes around 80 species of Echeveria.) It almost seems as if, practically speaking, this whole chromosome business doesn't really matter. (Reference 2)
How nature basically takes care of this ploidy problem for the average succulent breeder
They found that when two succulents with very different chromosome numbers came together to make a hybrid, either autosyndensis or allosyndesis occured. Autosyndesis is when chromosomes from both parents paired up because they were homologues, in other words, it creates a hybrid where some of its chromosomes that are paired came from the same parent. Allosyndesis is when they pair but they are not homologues, or is a hybrid from two different parents.
Autosyndesis
So, let's say you've got a diploid called S. allantoides (58 chromosomes) and then you've got a tetraploid called E. secunda (32 chromosomes) and they make a triploid hybrid. How does this happen? Well, the S. allantoides forms 29 pairs of chromosomes and the E. secunda forms 16 pairs of chromosomes. This matches the observed 45 chromosomes, indicating that autosyndesis is strongly preferential, where the chromosomes all match up with each other from the same parent. The hybrid has very fertile pollen. This example comes from reference 3.
Allosyndensis
If you had two diploid species, then during meiosis, you would only have one chromosome of each kind that could not pair with each other from the same parent. It would be forced to pair with the corresponding set from the second parent, resulting in allosyndensis. It is noted that there are never any more chromosome pairs observed than the number of the least-numbered chromosome parent.
What does this mean for me?
This all adds up to...succulents can take care of themselves. Even when faced with a partner of different n-values, they are able to sort themselves out. Hopefully, this post gave you some more insight into why it is possible for you to go cross-breeding all over the place, sashaying across genera boundaries and mixing up species for your own pleasure. It should also help you understand the complications of genetics and why, most often, you end up with a hybrid that is infertile (can't make seeds, but can propagate by leaf pullings). It all comes back to chromosomes.
I hope you enjoyed this week's Succulent Science Sunday! Please hit SUBSCRIBE to keep up-to-date on more succulent science!
References:
1) http://www.crassulaceae.com/crassulaceae.com/botanik/pflanzen/botanzeige_scan_en.asp?gnr=1610&scan=122880-2&cat=7&name=Echeveria
2) The Problem of Ploidy in Echeveria (Crassulaceae) I. Diploidy in E. Ciliata
Author(s): Charles H. Uhl
Source: American Journal of Botany, Vol. 69, No. 5 (May - Jun., 1982), pp. 843-854
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2442977
Accessed: 08-07-2018 03:52 UTC
3) Polyploidy, Dysploidy, and Chromosome Pairing in Echeveria (Crassulaceae) and Its Hybrids
Author(s): Charles H. Uhl
Source: American Journal of Botany, Vol. 79, No. 5 (May, 1992), pp. 556-566
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2444868
Accessed: 08-07-2018 03:52 UTC
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