B chromosome in Plantago lagopus Linnaeus, 1753 shows preferential transmission and accumulation through unusual processes

Living on the edge: morphological, karyological and genetic diversity studies of the Hungarian Plantago maxima populations and established ex situ collection

BACKGROUND

The analysis of genetic diversity of protected plant species can greatly support conservation efforts. Plantago maxima Juss. ex Jacq. is a perennial species distributed along the Eurasian steppe. The westernmost range edge of the species' distribution is located in the Pannonian basin, in Hungary where it is represented by a few, fragmented and highly endangered populations. We studied population diversity of all Hungarian range edge, natural populations, and one established ex situ population. One population from the centre of distribution (Kazakhstan) was implemented in the cpDNA haplotype study to compare the peripheral vs. central populations. We performed morphometric trait-based analysis, chromosome studies (morphometric analyses and FISH) and genetic diversity evaluations using inter simple sequence repeats (ISSR) and cpDNA trnL-trnF to evaluate differences between the in situ and ex situ populations as well as central vs. peripheral populations.

RESULTS

Our results showed no obvious morphological differences among the in situ and ex situ populations in the period between 2018 and 2020. One ex situ subpopulation develops flowers three years in a row from 2019, which is a favourable indicator of the introduction success. Hungarian populations are exclusively diploids (2n = 2x = 12). The karyogram consists of 5 metacentric and 1 acrocentric chromosome pair. Plantago maxima has one 35S and two 5S rDNA loci, located on the acrocentric chromosome pair. Eight variable ISSR primers yielded 100 fragments, of which 74.6% were polymorphic (mean H_e = 0.220). A high level of genetic variation within population was observed (92%) while the genetic differentiation among the populations was only 8%. STRUCTURE analysis revealed that the largest Kunpeszér population separated from the rest of the Hungarian populations, indicating a high rate of admixture among the other ones. Based on the trnL-trnF sequence analysis the Hungarian populations represent a single haplotype, which can indicate a reduced diversity due to isolation and recent population decline. By contrast, Kazakh population represents a distinct haplotype compared to the Hungarian samples.

CONCLUSIONS

The present study draws the attention to the high conservation value of the Plantago maxima populations from the westernmost range edge of the species' distribution.

Comprehending the dynamism of B chromosomes in their journey towards becoming unselfish

Investigated for more than a century now, B chromosomes (Bs) research has come a long way from Bs being considered parasitic or neutral to becoming unselfish and bringing benefits to their hosts. B chromosomes exist as accessory chromosomes along with the standard A chromosomes (As) across eukaryotic taxa. Represented singly or in multiple copies, B chromosomes are largely heterochromatic but also contain euchromatic and organellar segments. Although B chromosomes are derived entities, they follow their species-specific evolutionary pattern. B chromosomes fail to pair with the standard chromosomes during meiosis and vary in their number, size, composition and structure across taxa and ensure their successful transmission through non-mendelian mechanisms like mitotic, pre-meiotic, meiotic or post-meiotic drives, unique non-disjunction, self-pairing or even imparting benefits to the host when they lack drive. B chromosomes have been associated with cellular processes like sex determination, pathogenicity, resistance to pathogens, phenotypic effects, and differential gene expression. With the advancements in B-omics research, novel insights have been gleaned on their functions, some of which have been associated with the regulation of gene expression of A chromosomes through increased expression of miRNAs or differential expression of transposable elements located on them. The next-generation sequencing and emerging technologies will further likely unravel the cellular, molecular and functional behaviour of these enigmatic entities. Amidst the extensive fluidity shown by B chromosomes in their structural and functional attributes, we perceive that the existence and survival of B chromosomes in the populations most likely seem to be a trade-off between the drive efficiency and adaptive significance versus their adverse effects on reproduction.

Satellite DNA content of B chromosomes in the characid fish Characidium gomesi supports their origin from sex chromosomes

The origin of supernumerary (B) chromosomes is clearly conditioned by their ancestry from the standard (A) chromosomes. Sequence similarity between A and B chromosomes is thus crucial to determine B chromosome origin. For this purpose, we compare here the DNA sequences from A and B chromosomes in the characid fish Characidium gomesi using two main approaches. First, we found 59 satellite DNA (satDNA) families constituting the satellitome of this species and performed FISH analysis for 18 of them. This showed the presence of six satDNAs on the B chromosome: one shared with sex chromosomes and autosomes, two shared with sex chromosomes, one shared with autosomes and two being B-specific. This indicated that B chromosomes most likely arose from the sex chromosomes. Our second approach consisted of the analysis of five repetitive DNA families: 18S and 5S ribosomal DNA (rDNA), the H3 histone gene, U2 snDNA and the most abundant satDNA (CgoSat01-184) on DNA obtained from microdissected B chromosomes and from B-lacking genomes. PCR and sequence analysis of these repetitive sequences was successful for three of them (5S rDNA, H3 histone gene and CgoSat01-184), and sequence comparison revealed that DNA sequences obtained from the B chromosomes displayed higher identity with C. gomesi genomic DNA than with those obtained from other Characidium species. Taken together, our results support the intraspecific origin of B chromosomes in C. gomesi and point to sex chromosomes as B chromosome ancestors, which raises interesting prospects for future joint research on the genetic content of sex and B chromosomes in this species.

Origin, Behaviour, and Transmission of B Chromosome with Special Reference to Plantago lagopus

B chromosomes have been reported in many eukaryotic organisms. These chromosomes occur in addition to the standard complement of a species. Bs do not pair with any of the A chromosomes and they have generally been considered to be non-essential and genetically inert. However, due to tremendous advancements in the technologies, the molecular composition of B chromosomes has been determined. The sequencing data has revealed that B chromosomes have originated from A chromosomes and they are rich in repetitive elements. In our laboratory, a novel B chromosome was discovered in Plantago lagopus. Using molecular cytogenetic techniques, the B chromosome was found to be composed of ribosomal DNA sequences. However, further characterization of the chromosome using next generation sequencing (NGS) etc. revealed that the B chromosome is a mosaic of sequences derived from A chromosomes, 5S ribosomal DNA (rDNA), 45S rDNA, and various types of repetitive elements. The transmission of B chromosome through the female sex track did not follow the Mendelian principles. The chromosome was found to have drive due to which it was perpetuating in populations. The present paper attempts to summarize the information on nature, transmission, and origin of B chromosomes, particularly the current status of our knowledge in P. lagopus.