Signaling pathway could play a part inside the gynoecium differentiation in persimmon flowers. Within the genus Populus, the SDR was identified on chromosome 19 in distinctive species (Gaudet et al., 2008; Paolucci et al., 2010; Kersten et al., 2014; Geraldes et al., 2015). Genomewide association research (GWAS) revealed a homolog on the A. thaliana gene pair ARABIDOPSIS RESPONSE REGULATORFIGURE two | Single-gene sex determination enables turnovers between XY and ZW systems. (A) The feminizing sex switch (magenta box), i.e., ARR17 and MeGI in poplar and persimmon, respectively, is positioned outdoors in the sex-determining region (SDR, indicated by gray shading), mainly because a hairpin RNA-encoding Y-chromosomal sequence (blue box) controls its activity in trans via tiny RNAs, through a dominant repressing action (ARR17 inv. rep: ARR17 inverted repeats). (B) Intriguingly, IP Formulation exactly the same sex gene seems to be a sex-determining gene inside a ZW technique in white poplar (P. alba). Copies with the gene are identified within the SDR of this species, and its absence in the Z chromosome (dashed gray box) results in 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 powerful candidate for sex determination inside the closely related balsam poplars Populus balsamifera and Populus trichocarpa (Geraldes et al., 2015; McKown et al., 2017). Further analysis identified partial duplicates of ARR17 within the male-specific area with the Y chromosome (MSY) (M ler et al., 2020). Notably, these duplicates are present in aspens and balsam poplars, which represent two various EP medchemexpress sections from the genus, suggesting the possibility of a shared mechanism of sex determination. Regardless of these commonalities, phylogenetic evaluation indicated that the sexlinked ARR17 duplicates evolved independently (M ler et al., 2020; Zhou et al., 2020b). Lengthy read sequencing showed that the partial duplicates are arranged as inverted repeats, giving rise to smaller 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 fully functional males (M ler et al., 2020). This comprehensive sex reversal, in lieu of 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 read sequencing and de novo assembly of a female white poplar identified a W chromosome-specific contig with 3 full 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 therefore seems to become determined by a very simple presence/absence mutation of ARR17 (Figure 2B). Intriguingly, the single-gene-based mechanism of dioecy provides a simpleTABLE 1 | Candidate genes for sex determination with or without the need of functional validation in various 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.