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Fertilization
Fertilization within Drosophila occurs a bit differently compared to other organisms. In Drosophila, fertilization is internal and females store sperm Many post-mating barriers occur within fertilization that can exist at a variety of levels. These allow the process of unification with the gametes to occur. Sperm must successfully enter the female and be transported to the storage organs, the spermathecae or ventral receptacle. The sperm must stay alive until they are utilized by the female. The sperm later will be retrieved from the storage organs, activated, and then will enter an oocyte. It must pass through the reproductive tract in order to complete this process. Entrance of sperm to the egg must travel through the micropyle to trigger the formation of a normal zygote. A failure at any of these steps prevents fertilization. This is one of the reasons that many sperm are withheld in case the fertilization are not successful. In order to help assortative fertilization, there must be some degree of heritable specificity in the male and female components in the process. Fertilization barriers help to remove abnormal, non-viable sperm, or ejaculates to ensure successful fertilization. The male must also signal the female to keep his sperm alive thus allowing her to utilize them in fertilization for many more eggs. Since the female can hold onto sperm from one or many males, then the female must be able to detect and respond to the sperm differences. This will also lead to differential fertilization within the eggs. Having sperm from different males will cause different potential signals to be sent out. These can signal different receptors and will further complicate the fertilization process.

Both the sperm and non-sperm components of the ejaculate are highly different in Drosophila. Sperm in Drosophila species have a high levels of sperm length variability when compared to other animal taxa. They range from 0.32 mm in Drosophila persimilis to 58.29 mm in Drosophila bifurca. In the bifurca, the sperm are approximately 12 times longer than the male itself. There is little to no variation when analyzing individuals of the same Drosophila species. Species also differ in how much of the sperm tail enters the egg. These differences are important when analyzing post-fertilization isolation. Female sperm storage organs show interspecific variability as well; species also differ in which of those two organs, the spermathecae or the ventral receptacle, sperm are stored. These differences lead to whether or not assortative fertilization may be mediated by a mismatch between sperm morphology or storage organs within the females. In four different species of Drosophila, sperm differ considerably in length. The sizes of the storage sites within the females also differ in length. Females of Drosophila pachea and Drosophila wassermani store sperm only in the spermathecae. Drosophila nannoptera females use only the ventral receptacle. Morphological differences may end up deciding storage site, sperm viability or its reduction. This can also result from biochemical interactions that occur while the ejaculate passes through the female reproductive tract.

Extensive variation has been found within research in the chemistry and function of the non-sperm component of the ejaculate. More specifically, the accessory gland proteins or Acps of the ejaculate. Approximately 80 of these proteins are transferred to females at the time of mating. Only 9 of these proteins have been described based on function and chemistry. It is clear from these studies that this group of substances are used to alter female behavior. This alters female behavior by stimulating oviposition and delaying the time that the female will mate to another male and facilitate the storage of sperm in the female.

It is clear that within species there is considerable sequence polymorphism at the loci encoding these proteins and that the proteins show considerable sequence divergence between species as well as species specificity of their functions. In D. melanogaster, variation in four accessory gland proteins examined appears to be associated with displacement abilities in ejaculate competition experiments, although these experiments were not designed to detect any interaction with female genotype. Once the sperm is inside the female, there are a variety of reactions that must occur for successful fertilization. Some of these reactions occur within the female reproductive tract itself, but others involve the action of male-derived substances in other sites in the female. Evidence of the reactions inside the female reproductive tract is both direct and indirect. Acp36DE has been shown to localize at the entrance to the sperm storage organs. This will help to put all of the sperm into storage without any diversion.

Some reactions occur outside of the female tract come from three different protein: esterase 6, sex peptide and Acp26Aa. These small proteins are transferred quickly to the female hemolymph, even before copulation has terminated. They produce the same response in females. This is: increased oviposition and decreased receptivity to mating with another male. The 2 effects from sex peptides on female behavior are from the same unknown molecular target. The increase in oogenesis is mediated by the resultant increase in juvenile hormone synthesis in response to the sex peptide. An important direction for future research is the identification of the female targets of these proteins and the detection of variation in the targets that could provide mechanisms for differential fertilization. Species-specific morphological and biochemical features of the female tract are easily inferred from the larger size and longer duration of the insemination reaction mass. In house flies, female accessory gland secretions have been demonstrated to activate the sperm acrosome, enabling it to penetrate the micropyle.

Cleavage
Most insect eggs undergo superficial cleavage, wherein a large mass of centrally located yolk confines cleavage to the cytoplasmic rim of the egg. One of the fascinating features of this cleavage type is that cells do not form until after the nuclei have divided. Cleavage in a Drosophila egg in the image below. The zygote nucleus undergoes several mitotic divisions within the central portion of the egg. In Drosophila, 256 nuclei are produced by a series of eight nuclear divisions averaging 8 minutes each. The nuclei then migrate to the periphery of the egg, where the mitoses continue, at a progressively slower rate. During the ninth division cycle, about five nuclei reach the surface of the posterior pole of the embryo. These nuclei become enclosed by cell membranes and generate the pole cells that give rise to the gametes of the adult. Most of the other nuclei arrive at the periphery of the embryo at cycle 10 and then undergo four more divisions at progressively slower rates. During these stages of nuclear division, the embryo is called a syncytial blastoderm, meaning that all the cleavage nuclei are contained within a common cytoplasm. No cell membranes exist other than that of the egg itself.

Although the nuclei divide within a common cytoplasm, this does not mean that the cytoplasm is itself. When the nuclei reach the periphery of the egg during the tenth cleavage cycle, each nucleus becomes surrounded by microtubules and microfilaments. The nuclei and their associated cytoplasmic islands are called energids. Following cycle 13, the oocyte plasma membrane folds inward between the nuclei, eventually partitioning off each somatic nucleus into a single cell. This process creates the cellular blastoderm, in which all the cells are arranged in a single-layered jacket around the yolky core of the egg. Like any other cell formation, the formation of the cellular blastoderm involves a delicate interplay between microtubules and microfilaments. The first phase of blastoderm cellularization is characterized by the invagination of cell membranes and their underlying actin microfilament network into the regions between the nuclei to form furrow canals. This process can be inhibited by drugs that block microtubules. After the furrow canals have passed the level of the nuclei, the second phase of cellularization occurs. Here, the rate of invagination increases, and the actin-membrane complex begins to constrict at what will be the basal end of the cell. In Drosophila, the cellular blastoderm consists of approximately 6000 cells and is formed within 4 hours of fertilization.

After the nuclei reach the periphery, the time required to complete each of the next four divisions becomes progressively longer. While cycles 1–10 are each 8 minutes long, cycle 13, the last cycle in the syncytial blastoderm, takes 25 minutes to complete. Cycle 14, in which the Drosophila embryo forms cells (after 13 divisions), is asynchronous. Some groups of cells complete this cycle in 75 minutes, whereas other groups of cells take 175 minutes. Transcription from the nuclei (which begins around the eleventh cycle) is greatly enhanced at this stage. This slowdown of nuclear division and the concomitant increase in RNA transcription is often referred to as the mid-blastula transition. Such a transition is also seen in the embryos of numerous vertebrate and invertebrate phyla. The control of this mitotic slowdown appears to be affected by the ratio of chromatin to cytoplasm.

Gilbert, Scott F. “Early Drosophila Development.” ''Developmental Biology. 6th Edition.'', U.S. National Library of Medicine, 1 Jan. 1970, www.ncbi.nlm.nih.gov/books/NBK10081/.

Markow, Therese Ann. “Assortative Fertilization in Drosophila.” Proceedings of the National Academy of Sciences, National Academy of Sciences, 22 July 1997, www.pnas.org/content/94/15/7756.full.

Horvath, Barbara, et al. “The Genetics of Egg Retention and Fertilization Success in Drosophila: One Step Closer to Understanding the Transition from Facultative to Obligate Viviparity

.” Evolution: Journal of Organic Evolution, Society for the Study of Evolution, Feb. 2018, onlinelibrary-wiley-com.lib-proxy.radford.edu/doi/full/10.1111/evo.13411.

Tram, Uyen, et al. “Cleavage and Gastrulation in Drosophila Embryos.” Cleavage and Gastrulation in Drosophila Embryos, Encyclopedia of Life Sciences, 2002, sullivan.mcdb.ucsc.edu/pdf/35_Tram.pdf.