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Placozoans are small, flat, rather amorphous looking animals. They have a simple body plan with only six cell types and are without neurons and synapses

Nonetheless, placozoans show coordinated behavioral responses to chemicals emanating from their algal food, leading them to move toward food via ciliary gliding, externally digest the food, and take up the released nutrients.

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At its first discovery, it was obvious to many scientists that Trichoplax adhaerens presents significant differences from other known groups of organisms. However, a report classified it as the larval form of the cnidarian species Eleutheria krohi, which became the accepted hypothesis for the next several decades. In the 1970s, scientists, particularly the German protozoologist Karl Gottlieb Grell, renewed examinations of this species and demonstrated that the individuals examined were, in fact, adults. Trichoplax adhaerens has since been widely accepted as the sole representative of the phylum Placozoa, a name coined by Grell in 1971. Several recent morphological and molecular phylogenetic analyses have indicated that many additional species likely exist within this phylum, including several higher order taxonomic groups. To date, however, none of these have been formally described or named. Our understanding of the relationships of placozoans to other animal phyla remains in a state of flux

Early molecular phylogenetic analyses generally recovered them as the sister group to cnidarians or ctenophores. Subsequent studies, using much higher numbers of independent genetic markers, indicated that they occupy a basal position within metazoans as the sister group to the Eumetozoa, being placed between sponges (phylum Porifera) and all other multicellular animals.

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Spag proteins, however, do not form a genealogical gene super-family and the artificial grouping is based only on their function in spermatogenesis. Thus, in the absence of sister proteins, the different mouse Spag proteins, which vary in length between 90–2320 amino acids, cannot be subjected to phylogenetic analysis (Garcia-Reyero N, 2009). The same is true for Mns1 and Meig1. No related sister proteins exist that would allow phylogenetic orthology assignment. Thus, we conclude from similarity only that the placozoan Spag8, Mns1 and Meig1 proteins are homologs of the corresponding proteins in other species. Transcription of the sperm-oocyte recognition marker spag8 is linked to late or final stages of sperm in other animals. Although we have no functional data of spag8 in placozoans, the homology to other spag8 genes fuels the speculation of the production and storage of sperm during normal growth in the Placozoa

The latter seems to be the normal case for most bisexually reproducing animals, at least when they are dioecious. The storage of sperm allows a more rapid sexual response to a changing environment for example. The laboratory animals start to degrade when conditions become sub-optimal (Schwab K, 2003). Degrading animals reduce their lower epithelium and stop feeding.

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On balance, several new placozoan features and substantial evidence for recent bisexual reproduction in the Placozoa. Future studies will also be challenged by comparing developmental features between a yet unknown number of placozoan taxa

All of the above is of crucial importance for the steadily increasing number of developmental genetic studies that want to use Trichoplax as a basal metazoan model system.

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Garcia-Reyero N, Villeneuve DL, Kroll KJ, Liu L, Orlando EF, et al. Expression signatures for a model androgen and antiandrogen in the fathead minnow (Pimephales promelas) ovary. Environ Sci Technol. 2009

Schwab K, Patterson LT, Aronow BJ, Luckas R, Liang HC, et al. A catalogue of gene expression in the developing kidney. Kidney Int. 2003;64:1588–1604.

Shakib K, Norman JT, Fine LG, Brown LR, Godovac-Zimmermann J. Proteomics profiling of nuclear proteins for kidney fibroblasts suggests hypoxia, meiosis, and cancer may meet in the nucleus. Proteomics. 2005

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