Structure and activity of salivary gland chromosomes of Drosophila gibberosa

Apocrine secretion in the salivary glands of Drosophilidae and other dipterans is evolutionarily conserved

Apocrine secretion is a transport and secretory mechanism that remains only partially characterized, even though it is evolutionarily conserved among all metazoans, including humans. The excellent genetic model organism Drosophila melanogaster holds promise for elucidating the molecular mechanisms regulating this fundamental metazoan process. Two prerequisites for such investigations are to clearly define an experimental system to investigate apocrine secretion and to understand the evolutionarily and functional contexts in which apocrine secretion arose in that system. To this end, we recently demonstrated that, in D. melanogaster, the prepupal salivary glands utilize apocrine secretion prior to pupation to deliver innate immune and defense components to the exuvial fluid that lies between the metamorphosing pupae and its chitinous case. This finding provided a unique opportunity to appraise how this novel non-canonical and non-vesicular transport and secretory mechanism is employed in different developmental and evolutionary contexts. Here we demonstrate that this apocrine secretion, which is mechanistically and temporarily separated from the exocytotic mechanism used to produce the massive salivary glue secretion (Sgs), is shared across Drosophilidae and two unrelated dipteran species. Screening more than 30 species of Drosophila from divergent habitats across the globe revealed that apocrine secretion is a widespread and evolutionarily conserved cellular mechanism used to produce exuvial fluid. Species with longer larval and prepupal development than D. melanogaster activate apocrine secretion later, while smaller and more rapidly developing species activate it earlier. In some species, apocrine secretion occurs after the secretory material is first concentrated in cytoplasmic structures of unknown origin that we name "collectors." Strikingly, in contrast to the widespread use of apocrine secretion to provide exuvial fluid, not all species use exocytosis to produce the viscid salivary glue secretion that is seen in D. melanogaster. Thus, apocrine secretion is the conserved mechanism used to realize the major function of the salivary gland in fruitflies and related species: it produces the pupal exuvial fluid that provides an active defense against microbial invasion during pupal metamorphosis.

An overview of gene activity in the fat body of Drosophila gibberosa

In the larval fat body of Drosophila gibberosa, polytene chromosome structure and activity exhibit cytological differences from chromosomes of midgut and salivary glands. These differences include long-persisting puffs, transient puffs and long-persisting band modulations. Some early ecdysteroid-induced puffs are present in all three organs but few late puffs are present in the fat body. Comparative studies reveal, therefore, that late larval-early pupal puffing is enhanced in salivary glands relative to gut, fat body and Malpighian tubules. After the fat body breaks up in the prepupa, the rate of programmed cell death and the corresponding slow decline of chromosomal activity also differ from cell to cell and from other organs.

Developmental changes in midgut chromosomes of Drosophila gibberosa

In Drosophila gibberosa, differences between midgut and salivary gland chromosomes fall into two categories: tissue-specific band modulations which persist throughout the 90 h developmental period that we studied and tissue-specific puffs. Puffs that are common to both tissues tend to appear earlier in the midgut. Some major early ecdysteroid-induced puffs appear simultaneously in both tissues at the end of the third larval instar; however, the many late puffs that follow in the salivary glands are absent from the midgut. Intense puff activity in the early third larval instar midgut declines at the time of the hormonal pulse that initiates intense gene and secretory activity in salivary glands; the sloughing of midgut cells ensues.

Trans-regulation of ecdysterone-induced protein synthesis in Drosophila melanogaster salivary glands

A set of coordinately expressed genes is actively transcribed after a dramatic increase in the ecdysterone titer in late third-instar development of Drosophila melanogaster, as shown by the appearance of a number of puffs in salivary gland chromosomes and by the synthesis of a number of new proteins. Previous work has suggested that a product of the ecs gene, which is located within the 2B3-5 puff, is necessary for providing alterations in transcriptional activity at the sites of ecdysterone-dependent puffs. The experiments reported here were designed to determine whether the ecs gene's regulatory effect on puffing is confirmed by its regulatory effect on the synthesis of ecdysterone-inducible proteins (EIPs). The first series of experiments showed that in salivary glands in vivo ecdysterone induces 24 EIPs and in vitro induces 26 EIPs. The sets of polypeptides revealed are in good conformity. The second set of experiments demonstrated that mutations in the ecs locus disturb EIP synthesis: ecsl(1)t10 and ecsl(1)t143 mutations affect EIP synthesis to a lesser extent, while ecsl(1)t435 and ecsl(1)t324, as well as the 2B3-5 puff deficiency, prevent EIP synthesis completely. The experiments on dosage dependency revealed two EIPs whose rate of synthesis correlates with the dosage of the 2B3-5 X-chromosomal region. These EIPs are shown to be in fact small heat-shock proteins 23 and 28K, which are known to be encoded within the 67B puff and are under dual control--transient and developmental. The final set of experiments followed the 2B3-5 dosage dependency in vitro and showed that 15 EIPs display either an affected rate of synthesis or, mainly, a quicker induction time. Data obtained show that the ecs locus is trans-regulatory and that its product is necessary for spreading the effect of ecdysterone to other loci.