Flow cytometric, chromosomal and morphometric analyses challenge current taxonomic concepts in thePortulaca oleraceacomplex (Portulacaeae, Caryophyllales)

Abstract

Portulaca oleracea is a noxious annual weed of worldwide distribution in temperate to tropical climates. Its taxonomy has been treated in contradictory ways in the past. Various microspecies have been described, lumped into a single species by other authors. We re-examined the importance of seed size and ploidy variation, previously applied as the most important taxonomic characters, for systematic classification based on accessions from Europe, Asia, Africa and South America using flow cytometry, chromosome counting and morphometry. Sixteen microspecies and six transitional forms, covering the ploidy and seed character variation, proposed for the complex, were studied from 178 populations. Portulaca grandiflora was included as a reference species from outside the complex. DNA hyper-pentaploidy or hexaploidy were inferred for the majority of accessions which exhibited the full range of seed size. It is recommended that the only species of lower ploidy (either diploid based on x = 12 or tetraploid based on x = 12) encountered, P. nicaraguensis, should be separated from the P. oleracea complex as it deviates in base chromosome number and monoploid genome size. The frequency distribution of seed size was continuous and unimodal within the wild taxa of the complex and in pairs of taxa defined by testa sculpture. Seed size of DNA hexaploids was slightly negatively correlated with sample/standard fluorescence intensities. Our results conflict with the current microspecies concept. Possible reasons underlying the discrepancy are discussed and strategies for future systematic research are suggested.

Exploring C4–CAM plasticity within the Portulaca oleracea complex

“…It also presents a cosmopolitan distribution, occurring in most tropical and subtropical regions 36 . Variations in chromosome number 37 , vegetative and reproductive morphology 38 – 42 are already described between accessions of this species. Because of such high phenotypical plasticity, P. oleracea is sometimes referred to as a polymorphic species 33 , 37 , or even subdivided into different subspecies 43 , 44 or microspecies 45 – 47 that form a taxonomic aggregate or complex.…”

“…Variations in chromosome number 37 , vegetative and reproductive morphology 38 – 42 are already described between accessions of this species. Because of such high phenotypical plasticity, P. oleracea is sometimes referred to as a polymorphic species 33 , 37 , or even subdivided into different subspecies 43 , 44 or microspecies 45 – 47 that form a taxonomic aggregate or complex. The most up-to-date phylogeny detailing Portulaca infra-familiar relationships still refers to the traditional P. oleracea subspecies system to highlight that this clade is paraphyletic and clusters with other species, e.g.…”

“…To our knowledge, taxonomic reports list 19 subspecies/microspecies distinguished according to their seed size and coat ornaments 46 . On the other hand, P. oleracea is sometimes considered a polymorphic species, and due to its cosmopolitan distribution and high adaptability, it is somewhat expected to present high variability in morphological traits among populations, even forming a continuum 33,37,38,71 . Such plasticity might also affect seed attributes, making seed morphology and size alone inconclusive to differentiate subspecies, especially considering that hybrid subspecies have already been reported in mixed populations 33,37 .…”

“…On the other hand, P. oleracea is sometimes considered a polymorphic species, and due to its cosmopolitan distribution and high adaptability, it is somewhat expected to present high variability in morphological traits among populations, even forming a continuum 33,37,38,71 . Such plasticity might also affect seed attributes, making seed morphology and size alone inconclusive to differentiate subspecies, especially considering that hybrid subspecies have already been reported in mixed populations 33,37 . A comprehensive phylogeny including as many accessions of P. oleracea as possible will be needed to solve the species paraphyletic scenario, but its cosmopolitan distribution may prove to be a challenge to this goal.…”

First data on the taxonomic diversity of the Portulaca oleraceaaggregate (Portulacaceae) in Iran

“…Kim (2013) studied the pollen morphology in the broad-leaved and terete-leaved groups of the genus Portulaca. Although it has been verified that the morphological features of the seeds are a constant character in the progeny (Danin and Reyes-Betancort, 2006;Danin et al, 2008), recently Walter et al (2015) asserted that no correlation seems to exist between karyology and seed micromorphology. Danin and Raus (2012) distinguished and provided a determination key for the 19 microspecies within the P. oleracea aggregate.…”