BGS 614, Zeocriton 2, Zeo2

Since the zeocriton spike phenotype was first described by Hayes and Harlan (7), Zeo1 became the recommended locus symbol for various mutants at this locus. The semi-dominant dense spike variants at the complex Zeo1 locus exhibit a range of phenotypes associated with restricted elongation of the rachis internodes. Based on DNA sequencing, three distinct phenotypic groups of dense spike variants are associated with the Hordeum vulgare APELATA2 (AP2)-like transcription factor (HvAP2) locus (9, 10). The phenotypic expressions were historical assigned different locus symbols, but each group is associated with specific molecular changes in the HvAP2 transcript (9). Alleles at this locus are assigned locus symbols: The Zeo1 mutants exhibit the most extreme phenotypes, the Zeo2 variants have intermediate phenotypes, and the Zeo3 variants may show a lesser degree in shortening of rachis internodes (9). Molecular markers and/or pedigree information are necessary to separate variants assigned to the Zeo1 (BGS 082) and Zeo3 (BGS 742) groups (8). See BGS 082 for more information the alleles at the Zeo1 locus (9).

Zeocriton with no gene symbol (3, 7).
Zeocriton 2 = Zeo2 (6, 9).
Semidwarf mutant = Mo1 (11).
Rachis internode length QTL on 2HL = qSIL.ak-2H (11).
Cleistogamy 1 = cly1.b (8, 9, 13, 14).
Hordeum vulgare APELATA2 (AP2)-like transcription factor = HvAP2 (9. 10).

Monofactorial semi-dominant (2, 11).
Monofactorial semi-dominant (2, 11).
Located in chromosome 2HL (2, 11, 17); Zeo2 from Haruna Nijo Is in 2H bin 13 (2); qSIL.ak (Zeo2) from Kanto Nakate Gold is near SRS marker ABG613 (11); Zeo2 is closely to the Cleistogamy 1 (cly1.b) locus (10, 13, 14); Zeo2.c is associated with SNP markers 1_0404 to 1_0072 (positions 186.61 to 239.78 cM) in 2HL bins 12 to 14 of the Bowman backcross-derived line BW939 (3); the dense spike traits in the dsp1.a stock (likely Zeo2.c) is associated with SNP markers 1_0376 to 2_0561 (positions 209.87 to 247.86 cM) in 2HL bins 13 to 14 of the Bowman backcross-derived line BW277 (3); Zeo2 with the eog1.e gene from GSHO 285 (Ab 1380) is associated with SNP markers 1_1480 to 2_0895 (positions 173.50 to 209.91 cM) in 2HL bins 11 to 13 of the Bowman backcross derived line BW302 (3); Zeo2.ax (previously named dsp.ax) is associated with SNP markers 2_0064 to 2_0175 (positions 179.99 to 213.08 cM) in 2H bins 11 to 13 of the Bowman backcross-derived line BW270 (3); Zeo2 from Haruna Nijo in the ant22.1508 stock is associated with SNP markers 1_1346 to 2_0895 (positions about 165 to 209.87 cM) in 2H bins 11 to 13 of the Bowman backcross-derived line BW022 (3); the HvAP2 locus is near marker BOPA2_12_10579 (9), in 2H bin 13.

Many dense spike phenotypes with shortened rachis internodes have been described in the barley literature (12). Hayes and Harlan (7) identified a strong pyramid shaped spike with short internodes as zeocriton (little barley), but they reported that three genes controlled this trait in their cross to Zeocriton (8). Accession CIho 6238 is phenotypically to the Zeocriton cultivar. The progeny cross to the cultivar Svanhals (NGB 1482, PI 5474) best matched the inheritance pattern expected for cross segregating for the Zeo2 gene (7). Spikes of plants with the Zeo2 mutant are compact and remain strap-shaped because all rachis internodes are about the same length. Plants are slightly shorter and spikes contain 2 to 4 more fertile spikelets than those of normal sibs (21). Plants of the Bowman backcross-derived lines with Zeo2 alleles in the Zeo2.c and Zeo2.ax group have small anthers (2/3 of normal length) (5) and are associated with small lodicules (closed flowering or cleistogamy) (11, 14). The rachis internode length of plants in Bowman backcross lines for Zeo2 alleles averaged 3.3 mm compared to 4.5 mm for Bowman. In some field environments, the BW lines for Zeo2 gene had 1 to 2 more kernels per spike than Bowman and kernel weights were slightly lower. No effects of the Zeo2 gene on plant height and grain yield were observed (5 ). Cleistogamy or closed flowering was strongly associated with a gene for reduced rachis internode length (13), which was later identified as Zeo2 (9). The Zeo2 region of 2HL was associated with tolerance to low temperature injury (1) and Fusarium head blight infection (10, 16. 17). Cultivars with the Zeo2 mutant were demonstrated to have cleistogamous flowering pattern (11). The cleistogamous flower sheds its pollen before opening, forcing plants with this habit to be almost entirely autogamous. The lodicule in cleistogamous plants is atrophied (8). DNA sequencing showed that the Zeo1 mutants occur in a Hordeum vulgare APELATA2 (AP2)-like transcription factor, HvAP2. The dense spike and cleistogamous (small lodicules) phenotypes are a consequence of a perturbed interaction between microRNA 172 (Hv-miR172) and its corresponding binding site on the mRNA from the HvAP2 gene, which acts early in spike development to regulate turnover of HvAP2 mRNA (9, 10). The Zeo1 mutants occur in the last intron of HvAP2, the binding site of Hv-miR172, and prevent cleavage of the HvAP2 mRNA (9, 10).

A naturally occurring variant in the fourth exon of the HvAP2 locus (9), likely of occidental origin in two-rowed barley cultivars from Europe (9, 13); this variant was likely evaluated in Svanhals (NGB 1482, PI 5474) by Hayes and Harlan (7).

Phenotypically similar variants were isolated from several different accessions (2, 4, 7, 12). However, based on identical DNA haplotypes in the first exon of the HvAP2 gene, all of the variants have the same gene identified as Zeo2 (9. 10). Several cultivars from Northern Europe were identified as having the Zeo2 mutant including Golden Promise (PI 343079), Imperial (OUU352, PI 61340), and Plumage (OUU351, CIho 2511) (9, 10). The variants identified initially as having different origins include Zeo2.c from line 36Ab51 (GSHO 637), which was called “good zeocriton” (15), and Zeo2.ax from CIho 6880 and Haruna Nijo (3, 9); which likely is the Zeo2 allele from Golden Melon (OUJ808, PI 263410) as identified in Kanto Nakato Gold (OUJ 518) (8).

Zeo2.c in 36Ab51 (GSHO 637); Zeo2.c in Bowman (PI 483237)*4 (GSHO 3433); Zeo2.c in Bowman*4 (BW939, NGB 22368); Zeo2.ax from CIho 6880 (Ahang/Twisted Flag//Triple-awn Lemma) in Bowman*5 (BW270, NGB 22095); Zeo2.ax with ant22.1508 (Proanthocyanidin-free 22, GSHO 1635) from Haruna Nijo (NGB 13705) in Bowman*4 (GSHO 1841); Zeo2.ax with ant22.1508 in Bowman*6 (BW022, NGB 20430); Zeo2.c from 36Ab51 (GSHO 637, ‘good zeocriton’) in Bowman*4 (GSHO 3433, BW939, NGB 22368); Zeo2.c from a naked two-rowed accession from China [originally identified as dsp1.a (3)] in Bowman*7 (GSHO 1833, BW277, NGB 20561).

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J.D. Franckowiak. 2011. Barley Genet. Newsl. 41:193-194.
J.D. Franckowiak. 2017. Barley Genet. Newsl. 47:177-180.