Karyotypic stability of Fragaria (strawberry) species revealed by cross-species chromosome painting.

Chromosome karyotyping analysis is particularly useful in determining species relationships and the origin of polyploid species. Identification of individual chromosomes is the foundation for karyotype development. For Fragaria (strawberry) species, definitive identification of the individual chromosomes is extremely difficult because of their small size and similar shape. Here, we identified all chromosomes for 11 representative Fragaria species with different ploidy using a set of oligonucleotide-based probes developed in Fragaria vesca. Comprehensive molecular cytogenetic karyotypes were established based on the individually identified chromosomes. In addition, we used oligo probes to assign the 5S and 45S rDNA loci to specific chromosomes in 16 Fragaria species. We found that these Fragaria species maintained a remarkably conserved karyotype. No inter-chromosomal structural rearrangements at the cytological level were observed in any of the chromosomes among these species. Despite karyotypic stability and similarity, variations in the signal intensity of oligo probes were observed among the homologous chromosomes in several polyploid species. Moreover, most Fragaria species also showed differences in the distribution patterns of 45S and 5S rDNA. These data provide new insights into the origins of several polyploid Fragaria species.

Integrated Karyotyping of Woodland Strawberry (Fragaria vesca) with Oligopaint FISH Probes.

Chromosome identification is critical for many aspects of cytogenetic research. However, for Fragaria vesca, definite identification of individual chromosomes is almost impossible because of their small size and high similarity. Here, we demonstrate that bulked oligonucleotide (oligo) probes can be used as chromosome-specific DNA markers for chromosome identification in F. vesca. Oligos specific to entire pseudochromosomes in the draft genome of F. vesca were identified and synthesized as libraries. In all, we synthesized 6 oligo libraries corresponding to 6 pseudochromosomes of F. vesca. These libraries were amplified and labeled as probes for fluorescence in situ hybridization (FISH). Two rounds of multicolor FISH analysis were sequentially conducted on the same metaphase cells with each round including 3 probe libraries, which permitted simultaneous identification of all chromosomes of F. vesca. Moreover, 45S and 5S rDNA were mapped to chromosomes 1, 2, and 7, respectively. A karyotype of metaphase chromosomes was constructed, representing the first FISH-based molecular cytogenetic karyotype of F. vesca. Our study can serve as a basis for future comparative cytogenetic research through cross-species chromosome painting using bulked oligo probes and will facilitate the application of breeding technologies that rely on the identification of chromosomes in the genus Fragaria.