Long-term study of a subdioecious Populus ×canescens family reveals sex lability of females and reproduction behaviour of cosexual plants

Sex-determining region complements traditionally used in phylogenetic studies nuclear and chloroplast sequences in investigation of Aigeiros Duby and Tacamahaca Spach poplars (genus Populus L., Salicaceae)

Members of the genus Populus L. play an important role in the formation of forests in the northern hemisphere and are used in urban landscaping and timber production. Populus species of closely related sections show extensive hybridization. Therefore, the systematics of the genus is rather complicated, especially for poplars of hybrid origin. We aimed to assess the efficiency of application of the sex-determining region (SDR) in addition to the nuclear and chloroplast genome loci traditionally used in phylogenetic studies of poplars to investigate relationships in sections Aigeiros Duby and Tacamahaca Spach. Targeted deep sequencing of NTS 5S rDNA, ITS, DSH 2, DSH 5, DSH 8, DSH 12, DSH 29, 6, 15, 16, X18, trnG-psbK-psbI, rps2-rpoC2, rpoC2-rpoC1, as well as SDR and ARR17 gene was performed for 379 poplars. The SDR and ARR17 gene together with traditionally used multicopy and single-copy loci of nuclear and chloroplast DNA allowed us to obtain a clustering that is most consistent with poplar systematics based on morphological data and to shed light on several controversial hypotheses about the origin of the studied taxa (for example, the inexpediency of separating P. koreana, P. maximowiczii, and P. suaveolens into different species). We present a scheme of relationships between species and hybrids of sections Aigeiros and Tacamahaca based on molecular genetic, morphological, and geographical data. The geographical proximity of species and, therefore, the possibility of hybridization between them appear to be more important than the affiliation of species to the same section. We speculate that sections Aigeiros and Tacamahaca are distinguished primarily on an ecological principle (plain and mountain poplars) rather than on a genetic basis. Joint analysis of sequencing data for the SDR and chloroplast genome loci allowed us to determine the ancestors of P. × petrovskoe - P. laurifolia (female tree) × P. × canadensis (male tree), and P. × rasumovskoe - P. nigra (female tree) × P. suaveolens (male tree). Thus, the efficiency of using the SDR for the study of poplars of sections Aigeiros and Tacamahaca and the prospects of its use for the investigation of species of the genus Populus were shown.

The proposed role of MSL-lncRNAs in causing sex lability of female poplars

Labile sex expression is frequently observed in dioecious plants, but the underlying genetic mechanism remains largely unknown. Sex plasticity is also observed in many Populus species. Here we carried out a systematic study on a maleness-promoting gene, MSL, detected in the Populus deltoides genome. Our results showed that both strands of MSL contained multiple cis-activating elements, which generated long non-coding RNAs (lncRNAs) promoting maleness. Although female P. deltoides did not have the male-specific MSL gene, a large number of partial sequences with high sequence similarity to this gene were detected in the female poplar genome. Based on sequence alignment, the MSL sequence could be divided into three partial sequences, and heterologous expression of these partial sequences in Arabidopsis confirmed that they could promote maleness. Since activation of the MSL sequences can only result in female sex lability, we propose that MSL-lncRNAs might play a role in causing sex lability of female poplars.

Mapping the sex determination region in the Salix F1 hybrid common parent population confirms a ZW system in six diverse species

Within the genus Salix, there are approximately 350 species native primarily to the northern hemisphere and adapted to a wide range of habitats. This diversity can be exploited to mine novel alleles conferring variation important for production as a bioenergy crop, but also to identify evolutionarily important genes, such as those involved in sex determination. To leverage this diversity, we created a mapping population by crossing 6 Salix species (Salix viminalis, Salix suchowensis, Salix integra, Salix koriyanagi, Salix udensis, and Salix alberti) to common male and female Salix purpurea parents. Each family was genotyped via genotyping-by-sequencing and assessed for kinship and population structure as well as the construction of 16 backcross linkage maps to be used as a genetic resource for breeding and selection. Analyses of population structure resolved both the parents and F₁ progeny to their respective phylogenetic section and indicated that the S. alberti parent was misidentified and was most likely S.suchowensis. Sex determining regions were identified on Salix chromosome 15 in the female-informative maps for seven of the eight families indicating that these species share a common female heterogametic ZW sex system. The eighth family, S. integra × S. purpurea, was entirely female and had a truncated chromosome 15. Beyond sex determination, the Salix F₁ hybrid common parent population (Salix F₁ HCP) introduced here will be useful in characterizing genetic factors underlying complex traits, aid in marker-assisted selection, and support genome assemblies for this promising bioenergy crop.