Tephrocactus and multiplied chromosome sets

From the basic number to a cytotype

In the genus studied, the basic chromosome number is x = 11. A diploid has two such sets, giving 2n = 22. A tetraploid has four sets, 2n = 44. A cytotype is a particular cytogenetic form of a plant, so several cytotypes can occur under the same accepted species name. [Las Peñas et al. 2019]

Las Peñas, Kiesling and Bernardello combined chromosome counts, DNA measurements, morphology and chloroplast sequences. In T. molinensis, they found 2n = 44 and 88 in different specimens. Different levels also occurred in T. aulacothele (in the publication, T. weberi). This variation is part of the group's biology, rather than a simple division into one species with a small genome and another with a large one. [Las Peñas et al. 2019] [2-WWW]

Two historical names in one current species

In 2019, material identified as T. aoracanthus had 2n = 242. T. paediophilus with 2n = 319 was analysed separately. POWO now includes the latter name in the synonymy of T. aoracanthus. Each result remains linked to the historical identification and the specimen from which it was obtained. [Las Peñas et al. 2019] [2-WWW]

Cells and stomata

Di Dio and colleagues compared the anatomy of cultivated material. Cell and stomatal sizes changed with ploidy. Mean stomatal length was 25.36 µm in the T. alexanderi studied with 2n = 22, 54.38 µm in T. aoracanthus with 242 chromosomes, and 48.36 µm in material identified as T. paediophilus with 319 chromosomes. [Di Dio et al. 2022]

The relationship was comparative. Two materials with 2n = 88 differed in stomatal length, so the size of an individual structure does not replace chromosome counting. Lineage traits and development of the organ studied also play a role. [Di Dio et al. 2022]

Endopolyploidy within one plant

Some tissues contain cells that replicate DNA without the usual division producing two daughter cells. This process produces endopolyploidy. Cytometry can therefore detect several classes of DNA content in material from one specimen. A result concerning somatic nuclei is distinguished from the number of chromosome sets transmitted in the life cycle. [Las Peñas et al. 2019]

Plants with fragile stem-segment junctions reproduce vegetatively. A detached segment retains the genotype and can regenerate a whole plant. The authors considered the importance of this route in maintaining highly polyploid lineages, including those with an odd number of sets, where regular meiosis may be difficult. [Las Peñas et al. 2019]