Paternal Genome Elimination and accessory chromosomes

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Spermatid differentiation in citrus mealybug testes. Dots: cells containing the paternal genome that will be eliminated. The spermatids contain only the maternal genome.

Parent-of-origin effects are considered an important factor in the evolution and development of mammals and plants. Work on genomic imprinting has primarily focused on these two groups, yet reproduction in thousands of insect species involves an extreme form of imprinting known as “Paternal Genome Elimination” or PGE.

In this case, males recognise, silence and, during sperm formation, discard the chromosomes they inherited from their father. Only the mother’s chromosomes are inherited from the males. The molecular mechanisms responsible for parental origin recognition and process of elimination during sperm formation remain largely unknown.

To redress this, a research project (BNPGE) funded by a Marie Skłodowska Curie Independent Fellowship, aimed at exploring a unique genomic system found in the obscure mealybug Pseudococcus viburni: a “selfish” B chromosome that has acquired a way to escape from paternal genomic exclusion, therefore given itself the capability to be transmitted to the next generation regardless of parental origin.

Using comparative transcriptomic and genomic analyses, we are identifying the B linked genes and assessing whether these genes are involved in PGE. This system provides a rare chance to acquire novel insights into the epigenetic mechanisms underlying the recognition and exploitation of a chromosome’s parental origin, a widespread and evolutionarily conserved yet under-appreciated phenomenon.