The hnRNP A1 homolog Hrp36 is essential for normal development, female fecundity, omega speckle formation and stress tolerance in Drosophila melanogaster.

Hrp36/Hrb87F is one of the most abundant and well-characterized hnRNP A homolog in Drosophila and is shown to have roles in regulation of alternative splicing, heterochromatin formation, neurodegeneration, etc. Yet, hrp36 null individuals were reported to be viable and without any apparent phenotype, presumably because of overlapping functions provided by Hrp38 and related proteins. Here we show that loss of both copies of hrp36 gene slows down development with significant reduction in adult life span, decreased female fecundity and high sensitivity to starvation and thermal stresses. In the absence of Hrp36, the nucleoplasmic omega speckles are nearly completely disrupted. The levels of nuclear matrix protein Megator and the chromatin remodeller ISWI are significantly elevated in principal cells of larval Malpighian tubules, which also display additional endoreplication cycles and good polytene chromosomes. We suggest that besides the non-coding hsr omega-n transcripts, the Hrp36 protein is also a core constituent of omega speckles. The heat-shock-induced association of other hnRNPs at the hsr omega locus is affected in hrp36 null cells, which may be one of the reasons for their high sensitivity to cell stress. Therefore, in spite of the functional redundancy provided by Hrp38, Hrp36 is essential for normal development and for survival under conditions of stress.