Cucurbita maxima (Rimu) Genome
The Cucurbita genus contains several economically important species in the Cucurbitaceae family. Here we report high-quality genome sequences of C. maxima and C. moschata and provide evidence supporting an allotetraploidization event in Cucurbita. We are able to partition the genome into two homoeologous subgenomes based on different genetic distances to melon, cucumber and watermelon in the Benincaseae tribe. We estimate that the two diploid progenitors successively diverged from Benincaseae around 31 and 26 million years ago (Mya), and the allotetraploidization happened at some point between 26 Mya and 3 Mya, the estimated date when C. maxima and C. moschata diverged. The subgenomes have largely maintained the chromosome structures of their diploid progenitors. Such long-term karyotype stability after polyploidization has not been commonly observed in plant polyploids. The two subgenomes have retained similar numbers of genes, and neither subgenome is globally dominant in gene expression. Allele-specific expression analysis in the C. maxima × C. moschata interspecific F1 hybrid and the two parents indicates the predominance of trans-regulatory effects underlying expression divergence of the parents, and detects transgressive gene expression changes in the hybrid correlated with heterosis in important agronomic traits. Our study provides insights into polyploid genome evolution and valuable resources for genetic improvement of cucurbit crops.