Mulet (mlt) encodes a tubulin-binding cofactor E-like homolog required for spermatid individualization in Drosophila melanogaster.

Spermatogenesis in all animal species occurs within a syncytium. Only at the very end of spermatogenesis are individual sperm cells resolved from this syncytium in a process known as individualization. Individualization in Drosophila begins as a membrane-cytoskeletal complex known as the individualization complex (IC) assembles around the sperm heads and proceeds down the flagella, removing cytoplasm from between the sperm tails and shrink-wrapping each spermatid into its own plasma membrane as it travels. The mulet (mlt) mutation results in severely disrupted ICs, indicating that the mlt gene product is required for individualization. Inverse PCR followed by cycle sequencing maps all known P-insertion alleles of mlt to two overlapping genes, CG12214 (the Drosophila tubulin-binding cofactor E-like homolog) and KCNQ (a large voltage-gated potassium channel). However, since the alleles of mlt map to the 5'-UTR of CG12214 and since CG12214 is contained within an intron of KCNQ, it was hypothesized that mlt and CG12214 are allelic. Indeed, CG12214 mutant testes exhibited severely disrupted ICs and were indistinguishable from mlt mutant testes, thus further suggesting allelism. To test this hypothesis, alleles of mlt were crossed to CG12214 in order to generate trans-heterozygous males. Testes from all trans-heterozygous combinations revealed severely disrupted ICs and were also indistinguishable from mlt mutant testes, indicating that mlt and CG12214 fail to complement one another and are thus allelic. In addition, complementation testing against null alleles of KCNQ verified that the observed individualization defect is not caused by a disruption of KCNQ. Finally, since a population of spermatid-associated microtubules known to disappear prior to movement of the IC abnormally persists during individualization in CG12214 mutant testes, this work implicates TBCE-like in the removal of these microtubules prior to IC movement. Taken together, these results identify mlt as CG12214 and suggest that the removal of microtubules by TBCE-like is a necessary pre-requisite for proper coordinated movement of the IC.