Abstract
The ubiquitous presence of rRNA genes in nuclear, plastid, and mitochondrial genomes has provided an opportunity to use genomic markers to infer patterns of molecular and organismic evolution as well as to assess systematic issues throughout the tree of life. The number, size, location, and activity of the 35S rDNA cistrons in plant karyotypes have been used as conventional cytogenetic landmarks. Their scrutiny has been useful to infer patterns of chromosomal evolution and the data have been used as a proxy for assessing species discrimination, population differentiation and evolutionary relationships. The correct interpretation of rDNA markers in plant taxonomy and evolution is not free of drawbacks given the complexities derived from the lability of the genetic architecture, the diverse patterns of molecular change, and the fate and evolutionary dynamics of the rDNA units in hybrids and polyploid species. In addition, the terminology used by independent authors is somewhat vague, which often complicates comparisons. To date, no efforts have been reported addressing the potential problems and limitations involved in generating, utilizing, and interpreting the data from the 35S rDNA in cytogenetics. This review discusses the main technical and conceptual limitations of these rDNA markers obtained by cytological and karyological experimental work, in order to clarify biological and evolutionary inferences postulated in a systematic and phylogenetic context. Also, we provide clarification for some ambiguity and misconceptions in terminology usually found in published work that may help to improve the usage of the 35S ribosomal world in plant evolution.
Highlights
Ribosomes are universal cellular components found across all domains of life
Research suggests they represent the most critical macromolecular machine in living organisms, as they are trusted with carrying out protein synthesis in cells by converting information encoded within mRNA into peptides (Ojha et al, 2020)
The 5S intergenic spacers and the internal transcribed spacers (ITS) from the nuclear ribosomal 35S repeat have been proposed as nuclear standards for species identification (Gemeinholzer et al, 2006; Doveri and Lee, 2007; Gao et al, 2010), species delimitation (Müller et al, 2007) and DNA barcoding (Kress et al, 2005; Chase et al, 2007; CBOL Plant Working Group, 2009) in land plant lineages
Summary
Ribosomes are universal cellular components found across all domains of life. Research suggests they represent the most critical macromolecular machine in living organisms, as they are trusted with carrying out protein synthesis in cells by converting information encoded within mRNA into peptides (Ojha et al, 2020). The number, size, location and transcriptional activity of the 35S rDNA cistrons in plant karyotypes are some of the traditional cytogenetic landmarks reported in conventional karyotype descriptions (Battaglia, 1955) Their scrutiny has been useful to infer patterns of chromosomal evolution and the data have been used as a proxy for species discrimination, population differentiation and relationships at several evolutionary levels (Guerra, 2012). The terminology used by independent authors is somewhat vague, which often complicates comparisons This complexity does not appear to be fully appreciated by some systematic plant studies that use chromosomal, or cytological-based rDNA markers, alone or in combination with genomic data.
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