Population genomics of a biocontrol agent: Insights into post-introduction establishment in parthenium beetle

Biological control agents often experience demographic bottlenecks during introduction, which can reshape genetic diversity and inbreeding pattern influencing establishment and long-term ecological success in the introduced populations. The leaf-feeding beetle Calligrapha ( Zygogramma ) bicolorata , introduced in multiple countries across the globe to control the weed Parthenium hysterophorus , provides an opportunity to examine how introduction bottleneck and post-introduction breeding practices shape genomic variation and inbreeding patterns. We analyzed whole-genome variation in the introduced beetle population in India by sampling six regions encompassing its current distribution in the country. Using genome-wide variation data, we assessed population structure, genetic diversity, and inbreeding patterns across regions, and inferred historical changes in effective population size to reconstruct post-introduction demographic trajectories. The analyses reveal subtle genetic structure across regions, with overall genetic diversity relatively low compared to other invasive and biocontrol insects. Inbreeding patterns vary among populations, with some regions exhibiting higher cumulative runs of homozygosity than others. Notably, regions subjected to intensive propagation of beetle populations show elevated signature of inbreeding alongside reduced historical effective population size. These results underscore the dynamic genomic consequences of biocontrol introduction and subsequent breeding practices, providing insight into the evolutionary trajectories of introduced biocontrol agents.