Chromosome number data remain valuable today in systematics. Knowing which plants are polyploids and which are diploids allows for a better selection of individuals to be included in phylogenetic - DNA sequence - studies. Cytogeographic studies are useful in understanding distribution patterns and can help explain some morphological studies. While flow cytometry can result in large sample sizes and eliminates the need to collect buds for meiotic counts or transplanting for mitotic counts, it is critical that initial work begin with plants whose chromosome numbers have been determine by traditional means of squashing and counting. Once ploidy levels are known, then non-counting methods for determining ploidy level can be accurately applied. There are studies in the literature that did not do this and are seriously in error when what was assumed to be a diploid amount of DNA turns out to be a polyploid level.

Also, traditional methods of squashing and counting allow the researcher to detect chromosomal rearrangements, e.g. unequal translocations or dysploidy reductions in chromosome number with no change in the amount of DNA. These are not revealed by flow cytometry methods.

There are many species and genera of plants for which no chromosome count is known. There are countries from which very few counts have been reported. Therefore, there is still a great deal of traditional cytotaxonomic work to be done.

Thanks@John Semple; You have made the things clear. Can you please let me know the plants (as many as possible) whose chromosome counts have not been worked out till date. It will be nice, if you kindly name some Indian plants.

I have data on 11 chromosome counts for Solidago from India. All but one are probably for Solidago virgaurea or S. dahurica. I have not seen any of the vouchers. One report of 2n=27II was reported for Solidago canadensis but is likely for Solidago altissima from the Indian Botanic Garden, West Bengal. It would be useful to confirm that hexaploid S. altissima is present in Indian. Also possibly present as an introduction is S. chilensis (or S. microglossa); a diploid native to South American and introduced in tropical locations in the West Indies, Madeira Is., and probably other locations. Solidago altissima is naturalized in China (Beijing to Shanghai; Chen & Semple 2001, Flora of China) and in Japan and has been reported from scattered locations in Asia and elsewhere. It is native to eastern North America. Any additional chromosome reports for Solidago from India would be useful.

JCS

I would like to add to John's answer to say that one can infer quite a lot about plants sexual (and asexual) reproduction from chromosome numbers. Odd-number ploidy is often associated with meiotic disharmony in pollen and ovule production (i.e. it is difficult to have uniformly haploid gametophytes). There are various ways that plants can overcome the disharmony, including agamospermy. Sometimes pollen cannot even be produced. Sometimes pollen grains of uneven sizes are produced. With even-numbered ploidy, haplo-diploid miciro-mega gametophytic reproduction is the rule. Chromosome numbers are sometimes associated with the amount of genetic material in the cells, and with genetic load, phenotypic plasticity, and adaptability to stresses. One can infer a great deal, but then observations and experiments should follow. Cheers, Peter

@Peter,,thanks for your valuable suggestions.

Besides just chromosome number, it is also important to know the distribution of variation in chromosome numbers within a taxa.  Knowing the distribution of different ploidy levels can influence the interpretation of what the reports of a series of chromosome numbers means in terms of understanding what taxa and what ploidy levels are invasive in other parts of the world.  For example, while there are a number of reports of diploid (2n=18) S. altissima from North America, these are all for S. altissima var. gilvocanescens which is native to the Great Plains of central Canada and the central U.S.A.  What is the likelihood that S. altissima var. gilvocanescens was introduced into European gardens in the early to mid 1700s, when that part of North American had not yet been explored by botanists?  The likely source is eastern North America and likely Virginia as the source for S. altissima in European gardens.  Only hexaploids (2n=54) S. altissima is are known in Virginia.  Only hexploids of S. altissima have been reported from India, Taiwan, Japan, and Australia; i.e., it is the hexploid that is invasive outside of North America.  How many hexaploid reports for S. altissima are there from Europe?   If there are no hexaploid reports for S. altissima (or S. canadensis, which is only diploid in North America) from Europe, than what is the identity of diploid plants with lower hairy stems in Europe?  Which is more logical, S. canadensis var. hargeri or S. altissima var. altissima (with no known diploids in North America)?  The fact that diploids have been reported for S. altissima from North America should not be cited as a reason for the hairy stemmed diploids in Europe being S. altissima, because the morphology does not match diploid S. altissima var. gilvocanescens in North America.  One needs to known the distribution and the morphology of different races and different ploidy levels of a taxonomically challenging genus, if one wants to get the identity of invasives species correct.

Very useful!