Question 1
Generally, during the development of the fetus, the properties of organ teratogens depend on the timing of their presence. For example in Drosophila melanogaster which was used as a model organism to provide information for the timing and process of such events (Schlesinger, Ashburner, &Tissiere, 1982), heat shock appears to be as a teratogen only in the periods of wings and legs development. Specifically, only during this particular time period, heat shock inhibits the expression of the genes which play a crucial role in developing some body parts of larvae, like the wings or the legs. When the organism surpasses the heat shock, then the genes are able to be expressed again (Mednick S.A, Huttunen M.O, 2000). The aforementioned situation, is a environmentally induced error of development in Drosophila.
Regarding the gene mediated developmental errors, one good paradigm is the zfh-1 gene of Drosophila melanogaster. To begin with, in Drosophila, the differentiation of mesodermal cells depends on specific procedures during embryogenesis (Bate M, 1993). The concentration of dorsal protein is very important to the aforementioned differentiation, while it provokes the transcription of many genes (Schatten, 2008). The zfh-1 gene, encodes a zinc finger protein containing nine Cys2His2 and one homeodomain. The removal of this gene during embryogenesis didn’t affect the initial accumulation of mesodermic cells neither the differentiation of the majority of mesodermic cells. According to the research data, it rather provoked local errors in the position of the some cells (Lai Z.C, Rushton E., Bate M., Rubing G.M, 1993).
In another example of gene mediated developmental errors, participates the Toll gene. This one is responsible for the production of a transmembrane protein, important for the dorsal-ventral pattern formation (Anderson et al., 1985). In cases were none of the Toll genes were expressed, then muscle and motoneuron development experienced issues (Halfon M.S, Hashimoto C., Keshishian H., 1995). Another surprising feature of this analysis, was that the errors of Toll/+ embryos and larvae were difficult to identify, from the errors of Toll/Toll organisms. This detail is important and for its zygotic role in Drosophila (Halfon M.S, Hashimoto C., Keshishian H., 1995).
Question 2
The term “symbiosis” refers to the relationship between two distinct individual species and can occur with two forms. The first is parasitism, where the first organism lives on expense of another, while the other is mutualism and it refers to a beneficial relationship for both partners (Gilbert, 2013). In order for symbiotic relationships to endure in time, it’s important for the symbiotic mechanisms to be successful.
One example of such a symbiosis is observed in the marine fish N.nuchalis where luminous bacteria of the species P.leiognathi are the symbionts. Some fish species develop a specific structure named “light organ”, which is appropriate for the bacteria colonization (Urbanczyk et al. 2013). The tissues of the light organs are designed especially for reflecting and controlling the light produced by the bacteria with the use of the gas bladder.
The beginning of colonization in the light organ seems to begin early in the development of the fish (Dunlap, et al., 2007). Their migration then continues in an environment with a large amount of bacteria (Dunlap, et al., 2007). The formation of the light organ is preceded to the colonization of bacteria and thus there is a useful amount of time given for the light organ to develop properly (Dunlap, et al., 2007). From the aforementioned statement, the bioluminescence provoked by the existence of bacteria in a specific organ (light organ) in some fish species is a developmental symbiosis, which occurs early in juvenile fish.
The primary advantage of such a symbiosis is the survival in an environment where the source of light is decreased. The species N.nuchalis does indeed live in high depths, where light is not available. It is important to mention also, that the process of bioluminescence is controllable by oxygen concentration and bacteria growth. It is plausible that in cases of continuous luminance, those fish would be targets of numerous predators. The luminance is only produced when bacteria have an appropriate concentration of oxygen and can thus successfully grow.
The disadvantage of such a symbiosis could be the pathogen featutures of some bacteria strains that colonize around and through the light organ (Urbanczyk et al. 2013). Specifically, P.rosenbergii and P.jeanii are harmful to sponges mainly because of some pathogenic genes contained in their plasmids.
Reference:
Dunlap et al. (2007). Phylogenetic Analysis of Host-Symbiont Specificity and Codivergence in Bioluminescent Symbioses. Cladistics, 507-32.
Urbanczyk, H., Urbanczyk, Y., Hayashi, T., & Ogura, Y. (2013). Diversification of Two Lineages of SymbioticPhotobacterium, 8(12).
Gilbert F.S, (2013). Developmental Biology (10th ed.).
Halfon M.S, Hashimoto C., Keshishian H. (1995). The Drosophila Toll Gene Functions Zygotically and Is Necessary for Proper Motoneuron and Muscle Development. Developmental Biology, 151-167.
Lai Z.C., Rushton E., Bate M., Rubin G.M. (1993). Loss of function of the Drosophila zfh-1 gene results in abnormal development of mesodermally derived tissues. Proc.Natl.Acad.Sci. USA, 90, 4122-4126.
Martinez-Friaz M.L.,Frias J.L, Opitz J.M . (1998). Errors of Morphogenesis and Developmental Field Theory. American Journal of Medical Genetics, 291-296.
Mednick S. , Huttunen M.O. (2000). Lessons from The Wings of Drosophila. In C. R. Bergman L.R, Developmental Science The Holistic Approach (pp. 203-204).
Schatten, G. (2008). Current topics in developmental biology. Amsterdam: Elsevier Academic Press.
