Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - BNPGE (The escaping B chromosome: a tool for dissecting the mechanisms of Paternal Genome Elimination)

Teaser

Parent-of-origin effects are 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...

Summary

Parent-of-origin effects are 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. 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, the project aimed to explore a unique genomic system found in the mealybug Pseudococcus viburni: a “selfish” B chromosome that has acquired a way to escape from this genomic exclusion, therefore given itself the capability to be transmitted to the next generation regardless of parental origin.

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. Additionally, it has the potential to add new knowledge of applied value as gene inactivation under genetic conflict has been implicated in a wide range of human diseases and disorders.

The main objectives of the project were to dissect molecularly B chromosome escape from genomic exclusion, by 1) Obtain next generation sequencing data of populations of mealybugs carrying or not the B chromosome; 2) Identify candidate sequences specific to the B chromosome and potentially involved in B chromosome elimination; 3) Assess the epigenetic marks during B chromosome escape.

In conclusion, our project enabled to obtain the complete genome of B chromosome and its host mealybug, and identify potential candidates involved in its transmission to the next generation by circumventing Paternal Genome Elimination. Additionally, protocols were developed on an understudied insect system, yet of economic importance.

Work performed

During this project period, natural populations of Pseudococcus virburni from greenhouses in California and Scotland were collected, identified and screened for B chromosomes. We found lines carrying different numbers of B chromosome and selected a few populations to be established as laboratory colonies for the project. Using these laboratory populations, five genomic libraries carrying 2B, 1B and 0B were sequenced. By assembling the genome and performing coverage analysis, we identified sequences linked to different processes such as cell division and chromosome condensation that originated from the B chromosome.

Additionally, epigenetic markers were studied (antibody targeting histone modifications linked to chromosome condensation) and their presence assessed during spermatogenesis. As such, in collaboration with the lab’s Ph.D student and a visiting postdoc from the University of Montana, we established a working protocol allowing to observe these epigenetic marks for the mealybug system.

In summary, we obtained new genomic data of Pseudococcus viburni and its B chromosome, which allowed to identify several sequences linked to B chromosome transmission. This data will also be exploited in the future to understand other biological processes such as life history or resistance to pest management strategies. During the period of the project, we also used social media and the blogging platform Medium for outreach activities to share knowledge on peculiar sex determination and genetic systems.

Final results

B chromosome escape in mealybugs provides a unique system to understand how partial genomes are eliminated during sperm formation. Obtaining next generation sequencing data during the project enabled to access the genetic composition of this particular B chromosome. This data will be further exploited to understand the origin of B chromosome in this system and how it compares to other B chromosome carrying organisms.
In particular, the candidate genes identified on the B chromosome and potentially involved in sperm formation and chromosome condensation provide a starting point to find out the corresponding genes in the main genome that could be involved in the process of Paternal Genome Elimination.
This project allowed us to provide new genomic data on a plant pest (mealybug) and its B chromosome. The mealybug genome will be made available to the scientific community for continuing research that will help establish pest management strategies to control the species, a prevalent pest found on grapes in Europe and the rest of the world. Additionally, this project adds new knowledge of applied value as gene inactivation under genetic conflict has been implicated in a wide range of human diseases and disorders.

Website & more info

More info: https://isabellevea.wordpress.com/research-themes/paternal-genome-elimination-and-accessory-chromosomes/.