Mesozoan Orthonectids and Rhombozoans
Simple Animals or Confusing Mixtures of DNA
The term 'Mesozoa' was once used to describe two distinct animal phyla, and how they are related (to one another or to other phyla) is still unclear.
Mesozoan Biology
Mesozoa were once considered to be a single phylum in the animal kingdom, but are now classified as either members of the phylum Orthonectida or of the ‘sister’ phylum Rhombozoa. They are all very small parasites of marine invertebrates, consisting of a small number of cells and no internal organs – but they have very complicated ways of reproducing.
- Orthonectida - when they are free in the sea the adult orthonectid is not much more than a ciliated ball of cells with reproductive tissue inside. There are males and females, and when the male releases sperm it enters the female animal and fertilises the eggs. These eggs develop into small (also ciliated) larvae which leave their mother and enter a host animal (each species parasitizing a different group of marine invertebrate). Once inside the host animal the orthonectid becomes a blob of cells (called a ‘plasmodium’) which feeds at the host’s expense until returning to the sexual stage, when it leaves the host to begin the cycle again.
- Rhombozoa (or, more properly ‘Dicyemida’) are also very small parasites, but they are only found in the kidneys of squids and octopuses. The life-cycle is very complex, with one of the stages looking decidedly ‘worm-like’, suggesting a possible relationship with more advanced parasitic ‘worms’ (the platyhelminths of the phylum Platyhelminthes). These ‘rhombogen’ larvae (and the adult form) are very unusual in consisting of a fixed number of cells – indeed the exact number of cells is one of the things used to determine which species is being studied.
Some DNA and RNA studies have led to the conclusion that both phyla (the Orthonectida and the Rhombozoa) might be degenerate forms of ancient animals which were the ancestors of the modern platyhelminths, but results are very confusing. One fairly recent study (Noto and Endoh, in Biosystems volume 73, issue 1, January 2004) goes so far as to suggest that they might contain DNA and RNA from a number of different sources – some of their own and some from their host. If this turns out to be true then there are serious implications for the way molecular studies are used to help clarify the relationships between different animal groups.
Family Trees and DNA Transfer
- It is well known that members of the prokaryote kingdom (the bacteria and their allies) can transfer chunks of DNA (plasmids) from individual to individual – even between distinct species, and that viruses inject either DNA or RNA into host animals. This means that some of the genes that we find in an organism could well have come from somewhere other than a distinct ancestor.
- In the ‘higher’ animals (which have a distinct set of cells destined to produce eggs or sperm, sometimes referred to as the ‘germ line’) the transfer of DNA (genes) into body cells would have no effect on the DNA (genes) passed on to the next generation. Here we can still talk about ‘family trees’, but any study of the DNA or RNA of body cells might be misleading when trying to clarify relationships. What if a parasite could pass DNA from a mosquito to a horse – would that lead to the conclusion that horses and mosquitoes were closely related?
- Mesozoans (both of the phylum Orthonectida and phylum Rhombozoa) have reproductive cells which are not ‘protected’ from host tissues, and since they both feed by absorbing material from their host by sort of ‘sucking in’ material directly from cell to cell, there is the potential to transfer DNA (genes) from host to parasite.
In the case of the mesozoans, and maybe many other animal phyla, there is a need to be very cautious when using molecular research to give the definitive answers where the classification of animals is concerned?
DNA
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