This post is the second in a short blog series called “Know your Insect”. The images and descriptions are written by Entomology graduate students enrolled in a seminar of the same name.
By: Duverney Chaverra-Rodrigez
Reproduction is possibly the more dramatic event in insects’ life history. Numerous acts play in this show: fierce battles, nuptial gifts and fatal encounters are part of the amusement, deceive and tragedy behind the desire of reproduction in insects. Females are the major character of this play performing perhaps the hardest task: the production of a new generation within themselves. It could be that they lay up to 20000 eggs during their lives or give birth nymphs or larvae, females are responsible for population growth.
Such an important task deserved the attention of researchers on studying the female reproductive system, particularly the ovaries, to know more about its morphology and physiology and even to manipulate it to reduce or increase the levels of target insect populations.
A great case of this is the production of transgenic mosquitoes. Recent approaches to control the diseases transmitted by these vectors aimed to generate transgenic mosquitoes unpaired for parasite replication (Marelli et al 2007). However, it is frequently reported the difficulty to produce the transgenic mosquitoes in great numbers. Thus, some strategies to improve the number of transgenic mosquitoes seek to modify the eggs even before they are laid, when they are being produced in the ovaries (Peng 2011, Rasgon 2014).
This post will present information on ovaries and accessory reproductive structures in insects, emphasizing it in mosquitoes and its relationship with the research I do in mosquito transgenesis.
Ovary structure
In insects, ovaries are paired, musculated and highly tracheate organs, which are composed of ovarioles. The number of ovarioles per ovary is variable depending on the insect species, although a trend in increasing the number in more derived taxa has been noted by Gullan and Cranston (1994). Each ovariole is a tube made of epithelial tissue in which the eggs develop, they are connected to lateral oviducts through the pedicel.
In some insects (e.g. Ephemeroptera) the lateral oviducts open directly to the external cuticle through an ovipore. In most taxa however, a common oviduct that receives the two lateral oviducts has been developed.
The common oviduct could open to the external cuticle directly (as in Dermaptera) or opens (through the gonopore) to the genital chamber, which in addition, receives material and secretions from the spermathecal and the accessory glands (Büning 1994, Heming 2003).
Oocyte development
The anterior portion of the ovariole contains proliferating germ cells while the posterior part contains primary oocytes in different phases of development (but finally arrested in metaphase I; (Büning 1994, Heming 2003).
The oogonia in the terminal filament of one ovariole replicate and produce more oogonia and the primary oocytes. In some insects the oogonia produce additional cells known as nurse cells that provide the oocyte with nutrients thorugh cytoplasmic bridges during its development. Mesodermal cells surround each oocyte forming a follicle. Depending on follicle maturation ovarioles can be classified as panoistic (no nurse cells are produced) or meroisitic (with nurse cells) the latter is divided in polytrophic and telotrophic ovarioles depending if nurse cells have indirect or direct contact with the follicle respectively (Heming 2003). Panoistic ovaries are common in ancestral taxa compared to more derived taxa as noted by Gullan and Craston (1994). Mosquitoes ovaries are polytrophic with nurse cells in direct contact with the oocyte inside the follicle (Clements, 1992)
The Genital chamber, spermatheca and the accessory glands
In most insects the genital chamber is used as a place the copula and for egg fertilization. An ovipositor may be present or not. In tsetse flies (Glossinidae) the genital chamber is also used as an uterus that allows the development of larvae. The genital chamber receives sperm from the spermathecae and different substances from the accessory gland to facilitate eggs fertilization, and oviposition. The number of spermathecae is variable, in mosquitoes it may be from 1 to 3. The accessory glands are diverse and may be modified as venom glands in worker hymenopterans and “milk glands” in tsetse flies (Büning 1994, Clements 1992, Heming 2003).
The ovaries and transgenic mosquitoes
Mosquitoes are vectors of numerous parasites and pathogens causing millions of disease cases and deaths each year (WHO 2013). Recent approaches to control the diseases transmitted by these vectors aimed to generate transgenic mosquitoes unpaired for parasite replication. This mosquitoes would carry genes that express anti-parasitic molecules in the mosquito body, interrupting pathogen development. This idea has been investigated for more than one decade and now some transgenic lines of mosquitos have been produced (Kiszewski et al. 1998, Catterucia et al. 2000, Ito et al. 2002, Marelli et al. 2006). However, the transgenesis of mosquitoes carries out fitness costs associated mainly with endogamy due to the difficulties of producing high numbers of transgenic mosquitoes (Koenraadt et al. 2010, Meredith et a. 2014). Usually thousands of mosquito eggs are injected to get at least a couple adults that correctly integrated the transgene and may be used to establish a transgenic line. Thus, it is necessary to develop methods that are more efficient and suitable for using with different arthropod species (Rasgon 2014) since eggs are produced in ovaries. Would it be possible to efficiently deliver the genes of interest into the ovaries to improve transformation efficiency?
Peng et al. (2011) demonstrated that plasmidic DNA can be delivered into the ovaries efficiently to obtain embryos that transiently express heterogenous genes, or to knockdown gene expression levels. However it is necessary to produce mosquito lines that integrate those genes into the genome and a different approach is required.
My research is focused on creating a system to deliver the molecular machinery necessary to integrate genes into developing eggs in the ovaries using vitellogenin as a cargo transporter. If vitellogenic eggs steadily incorporate foreign genes, the efficacy of transgenesis will be higher since mosquito’s females produce dozens of eggs after each blood meal.
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