Genetic diversity and haplotype structure of the CelTOS gene in Plasmodium falciparum from Jazan Province, Saudi Arabia: implications for vaccine development and malaria elimination.

BACKGROUND

Malaria remains a persistent health challenge in southern Saudi Arabia, particularly along the border with Yemen. Understanding the genetic diversity and population structure of Plasmodium falciparum antigens is essential for tracking transmission dynamics and informing vaccine design. This study characterises population genetics of the Cell-Traversal Protein for Ookinetes and Sporozoites (CelTOS) gene in Jazan Province, a region of active transmission and frequent cross-border movement.

METHODS

We analysed 202 high-quality P. falciparum CelTOS coding sequences from 201 patients in Jazan. Diversity indices and neutrality tests were estimated in DnaSP, and demographic history was assessed using mismatch-distribution models (SSD, raggedness). Gene-wide dN/dS (ω) was derived from MEGA pairwise distances, and recombination was evaluated using PHI and GARD. Recombination-aware codon-based models (SLAC, FEL, MEME, and FUBAR) were implemented via Datamonkey. Population structure was examined using pairwise FST, AMOVA, principal coordinate analysis (PCoA), and PERMANOVA. Haplotype networks were stratified by nationality, parasite density, region, and season, and parasitaemia correlates were explored using multivariable regression.

RESULTS

The 726-bp CelTOS alignment contained 18 segregating sites and 27 haplotypes (Hd = 0.921; π = 0.0059). Gene-wide ω was ≈0.36, consistent with predominant purifying selection, with only a few codons showing episodic diversifying selection. The mismatch distribution was unimodal (SSD = 0.029; raggedness = 0.0487), and recombination tests indicated low-level intragenic recombination (PHI p = 1.44 × 10-5; Rm = 4). Pairwise FST values were essentially zero, and PERMANOVA showed a weak, non-significant regional structure. Dominant CelTOS haplotypes were shared across nationality, density, and regional and seasonal strata. In the main logistic model (PD4 vs. PD1-PD3), age (OR 1.03; 95% CI 1.00-1.06) and non-Saudi nationality (OR 2.21; 95% CI 1.02-4.81) showed modest associations with very high parasitaemia.

CONCLUSIONS

CelTOS diversity in Jazan is characterised by high haplotype but low nucleotide diversity within a largely panmictic, recombining parasite population. Extensive haplotype sharing across demographic and ecological gradients underscores strong gene flow, supporting cross-border, year-round control strategies and highlighting CelTOS as a relevant marker for vaccine development and molecular surveillance in this elimination setting.