Fig 1.
Cross direction-dependent incompatibility affects development of Capsella rubella and C. grandiflora hybrid seeds.
Percentage (A) and phenotypes (B) of aborted and non-aborted seeds of C. rubella, C. grandiflora and reciprocal hybrids of both species. Scale bars reflect 1 mm. (C) Percentage of germinated seeds of indicated crosses. (D) Seedlings of indicated crosses 10 days after germination. Seed area (E) and seed weight (F) of indicated crosses. Error bars show standard deviation. Significance was determined by t test analysis. * P < 0.05, ** P < 0.01 ns, not significant. In all graphs numbers on top of the bars correspond to number of analyzed seeds.
Fig 2.
Hybrid seed incompatibility between C. rubella and C. grandiflora correlates with endosperm cellularization defects.
Sections of C. rubella, C. grandiflora and reciprocal hybrid seeds at 4–7 days after pollination (DAP). Scale bar 100μm.
Fig 3.
C. rubella × C. grandiflora hybrid embryos are viable, revealing a major role of endosperm defects in hybrid seed incompatibility.
(A) C. rubella × C. grandiflora hybrid seeds at 13 days after pollination (DAP). (B) Section through seeds derived from crosses of C. rubella × C. rubella (left) and C. rubella × C. grandiflora (right) at 13 DAP. The sections reveal that hybrid embryos reach the torpedo stage at which development arrests. (C) Comparison of adult plants of all crosses (C. grandiflora × C. grandiflora (Cg), C. grandiflora × C. rubella (Cg × Cr), C. rubella × C. grandiflora (Cr × Cg), C. rubella × C. rubella (Cr)). Flower phenotypes of the respective genotypes are shown on top.
Fig 4.
Hybrid seed incompatibility is not a consequence of endosperm proliferation defects.
Endosperm nuclei numbers for each cross (C. grandiflora × C. grandiflora (Cg), C. grandiflora × C. rubella (Cg × Cr), C. rubella × C. grandiflora (Cr × Cg), C. rubella × C. rubella (Cr)) at indicated days after pollination (DAP). Three seeds per cross where counted. Error bars show standard error.
Fig 5.
Molecular response to reciprocal hybridizations of C. rubella × C. grandiflora is similar to interploidy hybridizations in Arabidopsis.
(A) Genes up- and downregulated in C. rubella × C. grandiflora reciprocal hybrid seeds compared to both parents overlap with genes deregulated in Arabidopsis interploidy seeds. The Arabidopsis osd1 mutant produces unreduced gametes, mimicking an interploidy hybridization when crossed with wild-type (WT). P values reflecting significance of overlap were calculated using a hypergeometric test. (B) Heatmap of expression log2 fold changes of selected AGL genes between samples. Capsella AGLs with several homologs in Arabidopsis are marked by small letters.
Fig 6.
C. rubella × C. grandiflora hybrid seed incompatibility involves multiple genetic loci.
(A) Frequency of F2 plants derived from crosses of C. grandiflora × C. rubella producing defined rates of seed abortion when backcrossed to C. rubella maternal plants. (B) Theoretical distribution of F2 plants producing defined rates of seed abortion when assuming that one maternal locus from C. rubella negatively interacts with two or three paternal loci from C. grandiflora. Experimental data correspond to data shown in (A). Significance of the observed distributions with the predictions of the models has been tested by Chi-square and P>0.05 is marked by an asterisk. Ns, non significant. (C) LOD scores for paternal C. grandiflora QTLs affecting abortion of C. rubella × C. grandiflora hybrid seeds. The purple line (top) represents the LOD score of each marker and the purple line (bottom) represents the effects. The red line represents the significance threshold as estimated by 1,000 permutations.