Bewick, A. | bioRxiv Science

bioRxiv2020

A derived ZW chromosome system in Amborella trichopoda, the sister species to all other extant flowering plants

Sex determination is poorly understood in plants. Amborella trichopoda is a well-known plant model for evo-devo studies, which is also dioecious (has male and female individuals), with an unknown sex determination mechanism. A. trichopoda is a "sex switcher", which points to possible environmental factors that act on sex, but populations grown from seed under greenhouse conditions exhibit a 50:50 sex ratio, which indicates the operation of genetic factors. Here, we use a new method (SDpop) to identify sex-linked genes from genotyping data of male and female individuals sampled in the field, and find that A. trichopoda has a ZW sex-chromosome system. The sex-linked genes map to a 4 Mb sex-determining region on chromosome 9. The low extent of ZW divergence suggests these sex chromosomes are of recent origin, which is consistent with dioecy being derived character in the A. trichopoda lineage. Our work has uncovered clearly formed sex chromosomes in a species in which both genetic and environmental factors can influence sex. One Sentence SummaryAmborella trichopoda, a dioecious species in which both genetics and the environment influence sex, possesses a pair of quite recently evolved ZW chromosomes.

plant biology↗

bioRxiv2020

Unusual predominance of maintenance DNA methylation in Spirodela polyrhiza

5-methylcytosine (5mC) is a modified base often described as necessary for the proper regulation of genes and transposons and for the maintenance of genome integrity in plants. However, the extent of this dogma is limited by the current phylogenetic sampling of land plant species diversity. Here, we report that a monocot plant, Spirodela polyrhiza, has lost CG gene body methylation, genome-wide CHH methylation, and the presence or expression of several genes in the highly conserved RNA-directed DNA methylation (RdDM) pathway. It has also lost the CHH methyltransferase CHROMOMETHYLASE 2. Consequently, the transcriptome is depleted of 24-nucleotide, heterochromatic, small interfering RNAs that act as guides for the deposition of 5mC to RdDM-targeted loci in all other currently sampled angiosperm genomes. Although the genome displays low levels of genome-wide 5mC primarily at LTR retrotransposons, CG maintenance methylation is still functional. In contrast, CHG methylation is weakly maintained even though H3K9me2 is present at loci dispersed throughout the euchromatin and highly enriched at regions likely demarcating pericentromeric regions. Collectively, these results illustrate that S. polyrhiza is maintaining CG and CHG methylation mostly at repeats in the absence of small RNAs. S. polyrhiza reproduces rapidly through clonal propagation in aquatic environments, which we hypothesize may enable low levels of maintenance methylation to persist in large populations. Significance StatementDNA methylation is a widespread chromatin modification that is regularly found in all plant species. By examining one aquatic duckweed species, Spirodela polyrhiza, we find that it has lost highly conserved genes involved in methylation of DNA at sites often associated with repetitive DNA, and within genes, however DNA methylation and heterochromatin is maintained during cell division at other sites. Consequently, small RNAs that normally guide methylation to silence repetitive DNA like retrotransposons are diminished. Despite the loss of a highly conserved methylation pathway, and the reduction of small RNAs that normally target repetitive DNA, transposons have not proliferated in the genome, perhaps due in part to the rapid, clonal growth lifestyle of duckweeds.

plant biology↗