Signaling pathway could play a function in the gynoecium differentiation in persimmon flowers. In the genus Populus, the SDR was identified on chromosome 19 in various species (Gaudet et al., 2008; Paolucci et al., 2010; Kersten et al., 2014; Geraldes et al., 2015). Genomewide association studies (GWAS) revealed a homolog from the A. GSK-3α web thaliana gene pair ARABIDOPSIS RESPONSE REGULATORFIGURE two | Single-gene sex COX-1 Purity & Documentation determination enables turnovers involving XY and ZW systems. (A) The feminizing sex switch (magenta box), i.e., ARR17 and MeGI in poplar and persimmon, respectively, is positioned outdoors of the sex-determining area (SDR, indicated by gray shading), because a hairpin RNA-encoding Y-chromosomal sequence (blue box) controls its activity in trans via little RNAs, via a dominant repressing action (ARR17 inv. rep: ARR17 inverted repeats). (B) Intriguingly, the identical sex gene appears to become a sex-determining gene inside a ZW method in white poplar (P. alba). Copies of the gene are discovered in the SDR of this species, and its absence from the Z chromosome (dashed gray box) leads to recessive loss of female and activation of male function.Frontiers in Plant Science | www.frontiersin.orgJanuary 2021 | Volume 11 | ArticleLeite Montalv et al.Sex Determination in Dioecious Plants16 (ARR16)/ARR17, which was named ARR17, as a robust candidate for sex determination in the closely related balsam poplars Populus balsamifera and Populus trichocarpa (Geraldes et al., 2015; McKown et al., 2017). Additional evaluation identified partial duplicates of ARR17 in the male-specific area of your Y chromosome (MSY) (M ler et al., 2020). Notably, these duplicates are present in aspens and balsam poplars, which represent two various sections of your genus, suggesting the possibility of a shared mechanism of sex determination. Despite these commonalities, phylogenetic evaluation indicated that the sexlinked ARR17 duplicates evolved independently (M ler et al., 2020; Zhou et al., 2020b). Extended read sequencing showed that the partial duplicates are arranged as inverted repeats, giving rise to little RNAs and apparently causing DNA methylation and silencing of the ARR17 gene, reminiscent in the OGI/MeGI method of persimmon (Figure 2A; Br tigam and Cronk, 2018;M ler et al., 2020). Most importantly, the functionality of ARR17 as a sex switch was demonstrated by CRISPR/Cas9mediated arr17 knockout in early flowering aspen lines, reverting females to completely functional males (M ler et al., 2020). This complete sex reversal, as an alternative to a reversion to hermaphrodites or neuters, demonstrates that ARR17 functions as a singlegene sex switch. Interestingly, white poplars (Populus alba) present a female heterogametic technique (ZW) (Paolucci et al., 2010). Extended study sequencing and de novo assembly of a female white poplar identified a W chromosome-specific contig with three total copies of ARR17 (M ler et al., 2020). Male white poplars usually do not carry any ARR17 sequences in their genome. Sex determination in white poplars as a result seems to be based on a basic presence/absence mutation of ARR17 (Figure 2B). Intriguingly, the single-gene-based mechanism of dioecy delivers a simpleTABLE 1 | Candidate genes for sex determination with or with out functional validation in diverse dioecious plant species. Species Actinidia deliciosa Original gene name Shy girl Friendly boy Asparagus officinalis SOFF aspTDF1 Diospyros lotus Fragaria virginiana, Fragaria chiloensis Ginkgo biloba MeGI GMEW RPP0W Gb_15883 Gb_15884.