Autotetraploid Origin of Chinese Cherry Revealed by Chromosomal Karyotype and In Situ Hybridization of Seedling Progenies

Abstract

Polyploidy is considered a driving force in plant evolution and diversification. Chinese cherry [Cerasus pseudocerasus (Lindl.) G.Don], an economically important fruit crop native to China, has evolved at the tetraploid level, with a few pentaploid and hexaploid populations. However, its auto- or allo-polyploid origin remains unclear. To address this issue, we analyzed the ploidy levels and rDNA chromosomal distribution in self- and open-pollinated seedling progenies of tetraploid and hexaploid Chinese cherry. Genomic in situ hybridization (GISH) analysis was conducted to reveal the genomic relationships between Chinese cherry and diploid relatives from the genus Cerasus. Both self- and open-pollinated progenies of tetraploid Chinese cherry exhibited tetraploids, pentaploids, and hexaploids, with tetraploids being the most predominant. In the seedling progenies of hexaploid Chinese cherry, the majority of hexaploids and a few pentaploids were observed. A small number of aneuploids were also observed in the seedling progenies. Chromosome 1, characterized by distinct length characteristics, could be considered the representative chromosome of Chinese cherry. The basic Chinese cherry genome carried two 5S rDNA signals with similar intensity, and polyploids had the expected multiples of this copy number. The 5S rDNA sites were located at the per-centromeric regions of the short arm on chromosomes 4 and 5. Three 45S rDNA sites were detected on chr. 3, 4 and 7 in the haploid complement of Chinese cherry. Tetraploids exhibited 12 signals, while pentaploids and hexaploids showed fewer numbers than expected multiples. Based on the GISH signals, Chinese cherry demonstrated relatively close relationships with C. campanulata and C. conradinae, while being distantly related to another fruiting cherry, C. avium. In combination with the above results, our findings suggested that Chinese cherry likely originated from autotetraploidy.

Keywords: Cerasus pseudocerasus (Lindl.) G.Don; Chinese cherry; GISH; autotetraploid origin; chromosomal karyotype; rDNA-FISH.

Figure 1

Chromosome numbers (2n) of self- and open-pollinated progenies in polyploid Chinese cherry. (A–D) 4x = 32, (A) Miyi 3; (B) Xichang 2; (C) Mengzi 3; (D) Anqiu 3. (E) Xichang 1 (E1: 4x = 32; E2: 5x = 40; E3: 4x + 1 = 33); (F) Bijie 7 (F1: 4x = 32; F2: 5x = 40); (G) Taihe 2 (G1: 4x = 32; G2: 6x − 1 = 47). (H) Nayong 1 (H1: 4x = 32; H2: 6x = 48; H3: 4x + 1 = 33); (I) Puding 3 (I1: 6x = 48; I2: 5x = 40; I3: 6x + 1 = 49); (J) Nanjing 2 (flowering cherry) (J1: 3x = 24, J2: 4x = 32); (K) Bazhong (K1: 4x = 32; K2: 5x = 40; K3: 6x = 48); (L) Luoyang 4 (L1: 4x = 32; L2: 5x = 40; L3: 6x = 48). Scale bar represents 3 μm.

Figure 2

Chromosomal karyotype (A) and idiogram (B) in polyploid Chinese cherry. m: metacentric, sm: submetacentric.

Figure 3

rDNA-FISH on metaphase chromosomes in polyploid Chinese cherry. (A) Xichang 1 (4x), (B) Mengzi 3 (4x), (C) Bijie 7 (4x), (D) Luoyang 4 (4x), (E) Bazhong (4x), (F) Bazhong (5x), (G) Puding 3 (5x), (H) Puding 3 (6x). The red and green signals indicate 45S and 5S rDNA sites, respectively. Scale bar represents 3 μm.

Figure 4

Karyotype (A) and idiogram in haploid complement (B) based on rDNA-FISH distribution in Chinese cherry.

Figure 5

GISH (A–E left) and karyotypes (A–E right) on metaphase chromosomes in Chinese cherry. Genomic probes were from C. pseudocerasus (A), C. campanulata (B), C. conradinae (C), C. serrulata var. lannesiana (D), and C. avium (E), respectively. Based on the traditional classification [9], C. pseudocerasus is assigned into section Lobopetalum subgenus Cerasus, C. campanulata and C. conradinae into sect. Serrula, C. serrulata var. lannesiana into sect. Sargentiella, and C. avium into sect. Cerasus subgenus Cerasus, respectively. Scale bar represents 3 μm.