Occurrence of subdioecy and scarcity of gender-specific markers reveal an ongoing transition to dioecy in Himalayan seabuckthorn (Hippophae rhamnoides ssp. turkestanica)

Abstract

Dioecy and the dynamics of its evolution are intensely investigated aspects of plant reproduction. Seabuckthorn (Hippophae rhamnoides ssp. turkestanica) is an alpine shrub growing wild in certain parts of western Himalaya. The previous studies have reported heteromorphic sex chromosomes in the species and yet marker-based studies indicate high similarity between the male and female genomes. Lack of information on sexual system in the species has further complicated the situation. A systematic study was thus undertaken to understand the sexual system in seabuckthorn and to discern the extent of similarity/dissimilarity between the male and female genomes by generating a large number of markers using amplified fragment length polymorphism and representational difference analysis. Floral biology and regular monitoring of species revealed the presence of polygamomonoecious (PGM) plants in most populations at a low percentage (~2-4%). PGM plants showed low pollen production and overall low fertility, suggesting a monoecy-paradioecy pathway at function. The results of the marker study demonstrated that there are limited differences between male and female genomes and these differences were not uniform across the populations in the Leh-Ladakh region, especially when the geographical distance increases. Results also suggest that a dynamic partitioning of genomes is operational between the two genders of seabuckthorn and differences are not homogenized across the populations. Both reproductive biology-based and DNA marker-based studies indicate that genders have separated recently. The present study proposes seabuckthorn as a promising model system to study evolution of dioecy and sex determination.

Fig. 1

a Outline map of India (not to scale) showing geographical region where study was conducted. b Distribution of 25 populations across three valleys, from where the plants were sampled (see Supplementary data Table S1 for details)

Fig. 2

Inflorescence of female (a), male (b), and polygamomonoecious plants (c) showing flowers (arrow). d–f Longitudinal section of bisexual flowers (A1G, A2G, and A4G) of PGM plants. Note that conduplication is incomplete in A2G (e) and A4G (f) flowers (arrow). an: anther, sg: stigma. Scale bars: a = 2 mm, b = 6 mm, c = 5 mm, and d–f = 500 μm

Fig. 3

Diagrammatic representation of HRMSSR locus (1–7016bp). a Gel profiles showing amplifications in male (left lane) and female (right lane) samples tested with different primer pairs (Supplementary data Table S3). Black bars show the male-specific amplification across populations with primer pairs HRML1 and HRML2 and HRML1 and HRML3. Lined gray block represents the amplicon of ~1200 bp in female with primer pair HRSS1 and HRML2 (see results for detail). b Location of a satellite, microsatellite, and transposons in HRMSSR locus. Dotted arrows show the position of 5 bp direct repeats “ACTCT” in female samples. Male has only one 5 bp sequence (ACTCT). c Organization of Ty3-gypsy-like and LTR-Ty1-copia-like retroelements at HRMSSR locus. The snapshots of alignments show conserved motifs (*). Black solid circles denotes the stop codons in translated HRMSSR sequence. Earlier known transposons sequences used for the alignment were obtained from the NCBI GenBank database. {Tobacco (Tto1): BAA11674, D83003; Tobacco (Tnt1): CAA32025, X13777; Orange (CIRE1): CAJ09951, AM040263; Drosophila (Copia): CAA26444, X02599; Tomato (TLC1.1): AAK29467, AF279585; Malus domestica (Ctrm1): FJ705357, XP00837822; Noccaea caerulescens (TNT1-94): JAU86839.1; Cajanus cajan: KYP58323.1; Phoenix dactylifera: XP_008779305.1; Populaus euphratica: XP_011027642.1; Ziziphus jujuba: XP_015894632.1; Hordeum vulgare: AY040832.1; Solanum lycopersicum: AAD13304.1; Arabidopsis thaliana: AAC69377.1; Zea mays: AAL76001.1}