Phenomenon of cell breakdown and phenotypic observation on surviving blades in interspecific hybridization progeny between Pyropia haitanensis and Pyropia dentata

In Pyropia haitanensis, meiosis occurs during the first two divisions of germinating conchospores, and then the latter formed a tetrad with diffierent genotype, finally the tetrad developed into a genotype chimeric blade. The heterozygous conchocelies was formed in the interspecific hybridization between Pyropia haitanensis and Pyropia dentata, which could produce conchospores after maturation, but the conchospores had serious cell breakdown at the early development stage of germination. Through the tracking observation of single conchospore germling, we found that cell breakdown occurred not only in the direct products of meiosis (dyad, triad and tetrad), but also in the mitosis when the number of germling cell was more than 4. In culture, most of the conchospores could form dyad after the first meiotic division, and then form triad or tetrad through the second meiotic division. When cultured for 4 days, 78.6% of the conchospores had developed into germlings containing 2–4 cells, and the rest were undivided conchospores; The percentage of germlings containing 2–4 cells that did not have dead cells was 99.7%, and 0.3% of individuals had dead cells. With the prolongation of culture time, most of the dyads developed into the triads and tetrads, while large-scale cells breakdown appeared in the triad and tetrad. After culture for 14 days, the percentages of triad and tetrad were 38.6% and 37.2%, respectively, in which 99.5% and 99.2% contained dead cells, respectively. In addition, 8.9% of the germlings could develop into individuals with more than 4 cells, but cell death also appeared in the subsequent mitosis. When cultured for 35 days, most of the cells of germlings had all died, still, about 1% of the germlings survived, but they did not develop from the complete tetrads, just developed from 1-2 surviving tetrad cells. After 45 days of culture, the surviving individuals could be divided into parental and recombinational type based on their morphology and color. The results confirmed that the hybrid breakdown occured in the early development stage of conchospores in interspecific hybridization of Pyropia, which could not only be used to assist in the more accurate identification of species, but also we could select new strains with recombination advantage from the surviving offspring.