Signaling pathway could play a part within 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 in the A. ALDH1 MedChemExpress thaliana gene pair ARABIDOPSIS RESPONSE REGULATORFIGURE 2 | Single-gene sex determination enables turnovers in between XY and ZW systems. (A) The feminizing sex switch (magenta box), i.e., ARR17 and MeGI in poplar and persimmon, respectively, is situated outdoors from the sex-determining region (SDR, indicated by gray shading), since a hairpin RNA-encoding Y-chromosomal sequence (blue box) controls its activity in trans by way of tiny RNAs, via a dominant repressing action (ARR17 inv. rep: ARR17 inverted repeats). (B) Intriguingly, precisely the same sex gene appears to become a sex-determining gene within a ZW method in white poplar (P. alba). Copies from the gene are identified within the SDR of this species, and its absence from 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 Caspase 3 Storage & Stability Dioecious Plants16 (ARR16)/ARR17, which was named ARR17, as a robust candidate for sex determination within the closely associated 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 of the Y chromosome (MSY) (M ler et al., 2020). Notably, these duplicates are present in aspens and balsam poplars, which represent two diverse sections of your genus, suggesting the possibility of a shared mechanism of sex determination. Regardless of these commonalities, phylogenetic analysis indicated that the sexlinked ARR17 duplicates evolved independently (M ler et al., 2020; Zhou et al., 2020b). Lengthy study sequencing showed that the partial duplicates are arranged as inverted repeats, giving rise to tiny RNAs and apparently causing DNA methylation and silencing from the ARR17 gene, reminiscent on the OGI/MeGI program 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 full 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 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 appears to become based on a uncomplicated 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 with no functional validation in unique 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.