Complex species (forms having different ploidy numbers) ?

Could someone explain the multilple chromosome counts for some “complex species” (forms have different ploidy numbers) and how does a species wind up having multiple chromosome counts? Also, how does one know which count a given plant would have?

R. acicularis (14, 28, 42, 56)

R. californica (14, 28)

R. canina (most are 35, but some forms are 42)

R. chinensis (most forms are 14, but some forms and species hybrids are 21, 28)

R. cinnamomea (14, 28)

R. eglanteria (35, 42)

R. moyesii (28, 42)

R. odorata (most are 14, but some forms and hybrids are 21, 28)

This is an expression of plant evolution. Primitive plant precursors had few chromosomes and one evolution path is chromosomes addition or fusion of two individuals (such are amphidiploids) that with time behave as balanced diploids and can fuse again.

A few tetraploids american species roses evolved this way from two diploids species.

That a species has different ploidies express changes are going with more or less autopolyploids forms that evolving over the millenaries may lead to new species.

“how does one know which count a given plant would have?”

By counting the chromosomes as does David.

But as variation of chromosome number is rather rare for balanced plants (i.e. not triploids) you can rather confidently deduct counts from ancestry.

I note Japanese researchers, who did a phylogenetic analysis of rose species native to that country using matK sequences, determined Rosa acicularis nipponensis is a “primitive taxon.” In their opinion, this diploid species should be reassigned to its original name Rosa nipponensis. Therefore, it’s unlikely there is a Rosa acicularis with 14 chromosomes.

Thank you for that explanation Pierre. It makes very good sense. Instead of asking how does one know which count a given plant would have I should have asked if when purchasing a species form that may have different ploidy numbers, do nurseries identify which variation they are selling.