Goldenrod chromosomes

All organisms, including goldenrods, have chromosomes in their cells.  Their chromosomes are organized in pairs, and the two chromosomes in a pair typically have the same genes in the same physical order, though the precise makeup of each gene (the sequence of bases in the DNA) often differs a bit between the genes (alleles) on those two chromosomes.  

When goldenrods reproduce, the sperm or egg carries only one chromosome from each pair because both types of gametes are the products of meiosis, a series of cell divisions that separate the pairs of chromosomes.  When egg and sperm meet, the resulting embryo (and plant, if the embryo survives and grows properly) will have pairs of chromosomes, one member of each pair from the mother, one from the father.  The number of chromosomes in the gametes is the haploid number (represented by n) and the number of chromosomes in an adult is the diploid number (2n), twice the haploid number.

Most species of goldenrods have nine pairs of chromosomes in each of their cells, a diploid number of eighteen.

But there are also goldenrod species with 36 chromosomes, some with 54, at least one with 90, and at least one that has 108 or 126.  Some species have 18, 36 or 54 chromosomes, some other species have 18 or 36, some other species have 36 or 54, and some other species have 18 or 54.  (Different published sources sometimes report different chromosome numbers for the same species.  They are not wrong!  They are telling us that we need to examine the chromosomes of more plants.)

You have probably already noticed that every number is 18 or a multiple of 18.  Somehow, some plants have ended up with more than the diploid number (2n) of chromosomes.  “Somehow” is a phenomenon called polyploidy, and it has occurred in essentially every lineage of vascular plants, and multiple times in the genus Solidago.

How can the number of chromosomes increase, and why in multiples of 18?

Here is one way: First, let’s say a goldenrod is producing egg cells in its ovary and sperm cells in its pollen grains, but for some reason, meiosis fails, and the gametes (egg and sperm) end up with 18 chromosomes (2n) instead of 9 chromosomes (n).  These would be the result of nondisjunction, a double negative that means the pairs didn’t separate when they were supposed to.  Then, let’s say that this plant is self-fertile (that is, pollen from its own anthers can pollinate ovules in its own ovaries).  If the diploid sperm fertilizes the diploid egg, the result is an offspring with 36 chromosomes (4n), a tetraploid plant.  This could be one way that some goldenrods ended up with 36 chromosomes.  This is one type of autoployploidy, polyploidy produced within a species.

Here is another way, which seems even less likely, but we have evidence that it has happened in many kinds of plants.  First, let’s say that nondisjunction happens in individuals of two different species of goldenrod.  We already know that many species of goldenrod can hybridize, but on rare occasions, they do so with diploid gametes, and the resulting 4n hybrid would have 36 chromosomes (tetraploid).  This is called allopolyploidy, polyploidy that results from hybridization.

Just in case you were wondering, polyploids are often sterile, but they can still reproduce vegetatively (a topic for a future post).

But it is also possible that they might be fertile and capable of producing seeds.  

But could tetraploid and diploid goldenrods breed with each other?  Let’s look at the chromosome numbers for a clue.  If a plant with 18 chromosomes crossed with a plant with 36 chromosomes, and if both produced gametes by meiosis, the result would be 9 + 18 = 27.  That’s a triploid number (3n), and no goldenrods have yet been found with that number of chromosomes in the adult plant.  Triploids are rarely fertile, and you have seen a triploid plant if you have ever eaten seedless grapes.  They can’t produce fully-formed seeds because triploids produce weird gametes (if they produce any at all) that rarely fuse. 

But perhaps you noticed that 2 x 27 = 54, and there are goldenrods with 54 chromosomes.  Maybe (I’m just speculating here) plants with 18 and 36 chromosomes each produced gametes by nondisjunction, so that the gametes had 18 chromosome from the 18 chromosome parent, and 36 chromosomes from the 36 chromosome parent.  The result could be 18 + 36 = 54.  Several species of goldenrods have plants with 54 chromosomes, and at least three species appear to have only 54 chromosomes (6n or hexaploid).

There are at least two species of goldenrod with 90 chromosomes (10n) and at least one with 108 or 126 (12n or 14n).  Polyploidy has gone crazy in these species, but still the numbers are multiples of 18.  These species (S. faucibus, S. lancifolia, and S. glomerata) all live in the southern Appalachians of Kentucky, Virginia, North Carolina and Tennessee.  Is there something about that geography, bedrock, soil, climate, evolutionary history, etc., that has produced more polyploid descendants than elsewhere?  I suspect so, but don’t know how to figure out why.

In contrast, all the species in eastern Asia, whether on the continent, peninsulas, islands, or mountains, have 18 chromosomes, the typical diploid number in Solidago.  Is there something about that region that has limited the occurrence of polyploidy?  I wonder…

One more thought about the origin of polyploid hybrids.  There is a polyploid species of cordgrass (genus Spartina) in Great Britain that seems to have arisen not through fertilization, but through merging of cells in the underground portions of the plants (roots and/or rhizomes).  These polyploids are fertile, breeding well amongst themselves, but not with any of the other cordgrass species in the saltmarshes where they live, all of which are diploid.  

Could any of the goldenrod polyploids be the result of underground mergers?  They have abundant roots and rhizomes, and they can grow vigorously in crowded stands.  I don’t know, but I’ll just throw this hypothesis out there.

But if goldenrods with different numbers of chromosomes cannot interbreed, then those “species” that have individuals with two or three different numbers of chromosomes might actually be different species, if we define species as groups of organisms that can successfully interbreed.  If so, we might be able to distinguish these species only by checking their chromosome numbers, not something that works well with a field guide to wildflowers, or even a dichotomous key intended to be used by experts.  If you already thought it was difficult to identify the different species of goldenrods, it might be a whole lot more difficult than you thought.

Suggested reading: 

John C. Semple 2016. An intuitive phylogeny and summary of chromosome number variation in the goldenrod genus Solidago (Asteraceae: Astereae). Phytoneuron 2016-32: 1–9.