Evolutionary success of a parasitic B chromosome rests on gene content

Supernumerary (B) chromosomes are dispensable genomic elements found in most kinds of eukaryotic genomes. Many show drive mechanisms that give them an advantage in transmission, but how they achieve it remains a mystery. The recent finding of protein-coding genes in B chromosomes has opened the possibility that their evolutionary success is based on their genetic content. Using a protocol based on mapping genomic DNA Illumina reads from B-carrying and B-lacking individuals on the coding sequences of de novo transcriptomes from the same individuals, we identified 25 protein-coding genes in the B chromosome of the migratory locust, 15 of which showed a full coding region. Remarkably, one of these genes (apc1) codes for the large subunit of the Anaphase Promoting Complex or Cyclosome (APC/C), an E3 ubiquitin ligase involved in the metaphase-anaphase transition. Sequence comparison of A and B chromosome gene paralogs showed that the latter show B-specific nucleotide changes, neither of which putatively impairs protein function. These nucleotide signatures allowed identifying B-derived transcripts in B-carrying transcriptomes, and demonstrated that they show about similar frequency as A-derived ones. Since B-carrying individuals show higher amounts of apc1 transcripts than B-lacking ones, the putatively higher amount of APC1 protein might induce a faster metaphase-anaphase transition in spite of orientation of the two B chromosome chromatids towards the same pole during metaphase, thus facilitating B chromosome non-disjunction. Therefore, apc1 is the first protein-coding gene uncovered in a B chromosome that might be responsible for B chromosome drive.\n\nSignificance StatementThe genome of the migratory locust harbors a parasitic chromosome that arose about 2 million years ago. It is widespread in natural populations from Asia, Africa, Australia and Europe, i.e. all continents where this species lives. The secret for such an extraordinary evolutionary success is unveiled in this report, as B chromosomes in this species contain active protein-coding genes whose transcripts might interfere with gene expression in the host genome (the A chromosomes), thus facilitating B chromosome mitotic and meiotic drive to provide the transmission advantage which grants its success. One of the B-chromosomal genes (apc1) codes for the large subunit of the Anaphase Promoting Complex or Cyclosome (APC/C) whose expression might provide a mechanistic explanation for B chromosome drive.