Mutant/mutation
The mutant lacks expression of Pb22

Protein (function)
PBANKA_0305900 (Pb22), which is predicted to encode a sexual-stage protein of 218 amino acids (aa) with a molecular weight of 22 kDa. Pb22 has a putative signal peptide (aa 1–18), suggesting that it might be secreted. Pb22 did not contain any domains of known functions, while the SMART program (http://smart.embl-heidelberg.de/) detected two low-complexity regions spanning aa 63–75 and aa 166–185, which contain compositionally biased amino acids. P22 is highly conserved among Plasmodium species. BLAST search also identified a homolog of this protein in Hepatocystis, which is considered to be descended from Plasmodium with the loss of blood schizogony.

Phenotype 
Normal numbers of mature male and female gametocytes are produced. Pb22-KO male gametocytes showed a ~89% reduction in the exflagellation centers formed. Normal formation (and egress) of female gametes. Strongly reduced ookinete formation. No oocyst formation.
The major defects in the pb22-KO line appeared to be at the egress step since 78.9% of the microgametocytes in the pb22-KO line were in the stage of aborted exflagellation with the RBC membrane retained. For microgametocytes with aborted exflagellation, 30% failed to assemble the axonemes and 48.9% were egress-defective. 
Cross-fertilization experiments showed that the pb22-KO line was defective only in male fertility wheras female fertility was not affected.

Other mutants