Distribution of B chromosomes in Prospero autumnale associates with higher reproductive performance in southern Iberia.

B chromosomes are supernumerary, dispensable chromosomes present in many eukaryotic genomes and are often considered as genomic parasites. However, their distribution and phenotypic effects in natural populations remain incompletely understood, particularly at local scales. Here, we surveyed 14 natural populations of the plant Prospero autumnale (L.) Speta (Asparagaceae) in southern Iberia and detected B chromosomes in only four of them. We quantified vegetative and reproductive traits and found that, at the population level, B-carrying populations exhibited significantly higher reproductive output, including increased seed number and fertility, than populations lacking Bs. However, we did not find significant differences between individuals with and without B chromosomes in populations that carry Bs. In contrast, no consistent differences emerged for vegetative traits. These findings suggest that B chromosomes tend to persist in populations with greater reproductive performance, consistent with their maintenance in benign environments where hosts can better tolerate their parasitic effects. Overall, our study sheds light on the ecological factors influencing B chromosome distribution and highlights the association between genomic parasites and host reproductive success.