Phylogeography and genetic structure of avian isospora based on mitochondrial COI gene sequences from 17 countries.

Isosporiasis is caused by apicomplexan protozoa of the genus Isospora, transmitted primarily via the fecal-oral route. While Isospora infects various vertebrates, infections are particularly notable in birds. However, the phylogeography and genetic structure of avian Isospora remain poorly documented. In total, 273 selected mitochondrial cytochrome c oxidase subunit I (COI) sequences belonging to avian Isospora species were retrieved from NCBI and subsequently analyzed to understand their phylogeographic patterns. Overall, 119 haplotypes, predominated by Hap_2, were identified across 17 countries. The dataset exhibited high haplotype diversity (Hd = 0.944) and low nucleotide diversity (π = 0.009). Significantly negative values of Tajima's D (-2.646) and Fu's Fs (-203.014) are compatible with recent demographic expansion under a single‑species assumption, though the presence of cryptic species cannot be excluded. Analysis of molecular variance (AMOVA) showed that 95.26 % of the total genetic variation was attributable to within-population differences suggesting substantial gene flow among most continental populations. Pairwise FST values were low between Europe and North America (0.017) and between Oceania and Europe (0.051, non-significant), but higher between Africa and other continents (0.24-0.34), indicating relative isolation of African lineages In summary, we report substantial haplotype diversity, a dominant cosmopolitan lineage, and contrasting patterns of connectivity - high gene flow among most regions but significant differentiation involving Africa - based on available COI sequences.