Cytogenetic and cytometric analyses in artificial intercytotypic hybrids of the emergent orchid model species Erycina pusilla

Erycina pusilla is considered a potential model organism for orchids, because of several advantageous features, such as short juvenile period, low chromosome number and all year round blooming. Two different chromosome numbers (2n = 10 and 2n = 12) are reported for E. pusilla, which suggests two cytotypes. To reveal the genome homology between these two cytotypes, we generated hybrids from the intercytotypic reciprocal crosses (2n = 10 × 2n = 12 and 2n = 12 × 2n = 10), and applied 4′,6-diamidino-2-phenylindole staining, genomic in situ hybridization and flow cytometry for genomic and cytogenetic analysis. The parental genomes showed high similarity both in genomic composition and content. The hybrids displayed a chromosome number of 2n = 11 in mitotic cells. Moreover, five bivalents and one univalent were observed at meiotic metaphase I stage. We observed meiotic synaptic behavior and found homeologous pairing with unpaired loops between parental chromosome pairing segments. These results demonstrated that chromosome rearrangement events have occurred between parental cytotypes during evolution. This study also illustrated the genome homology and homeologous pairing at pachytene phase, indicating that the chromosome number variation of two cytotypes mainly resulted from chromosome rearrangements, not changes in genomic constitution.