The intracellular bacterium (the symbiont) was assimilated into the larger host cell; the bacterial partner transferred many bacterial genes to the host’s nucleus, which resulted in the loss of independence, and new metabolic properties for the host. In most of these interactions, the larger host protoctist takes into its cells a smaller symbiont, instead of being eaten or killed is used whilst alive as a source of new materials or properties. The most common interaction is between heterotrophic protoctists (forams and ciliates) and algae.
The acquisition of chloroplasts, by endosymbiosis was the next innovation for sexual groups of protoctists. This acquisition allowed a switch to autotrophy and once they could photosynthesise they had a abundant energy source (light). It also provided oxygen for aerobic metabolism and new biosynthetic abilities. Chloroplasts were not required from cyanobacterium (primary endosymbiosis which only happened once), but from another eukaryote by a process of secondary endosymbiosis (second example of mutualism). (see p34, Fig1.28, Ridge, 2001)
Parasitism is the second type of symbiosis. Parasitism is a type of symbiotic association in which one partner benefits but the other is exploited (for food and shelter) and disadvantaged (Glossary, S204). People see parasites as disease agents but in fact many parasites cause no detectable harm to their hosts, so the longer the parasitic association, less harm it causes to the host. There are many associations mainly involving ciliates or flagellates, where protoctists live in animal guts and aid (help with their digestion) digestion.
Each specie of parasite tends to be highly specialized so that it can associate with only one or a few host species and with particular tissues or sites in hosts. Such high degrees of specialization evolved from general associations, for example with several host species or many tissues within one specie. So what happens is that different genetic strains of a generalized parasite became increasingly specialized to a particular host or tissue and so increasingly isolated from other strains. The end result has been the evolution of many specialized parasites from a small number of generalists.
So host specialization by parasites is a major cause of diversification within protoctists, as well as in bacteria and animals. Although metabolic and structural innovations occurred relatively early in protoctist evolution, diversification due to parasitism occurred later because it depended on the evolution of plant and animal hosts.
One example of parasitism is Malaria, which is caused by protoctist agents (Plasmodium spp.) and is the most widespread infectious diseases of human beings. Their life cycle provides us with an example where a unicellular protoctists passes threw several stages, each with different morphology and function.
There are two hosts: human being, where one stage of the life cycle occurs in the liver cells (haploid stages) and another in the red blood cells (diploid phase); and mosquito (anopheles spp.) (diploid phase). The mosquito picks up the parasite when it feeds blood from an infected person. Next sexual reproduction occurs in the mosquito’s stomach and meiosis follows cell fusion so that for most of its life cycle plasmodium exists as haploid amoeboid cells. Now the parasite has passed from the gut to the saliva glands and its transmitted in the saliva.
A second example of paratism is Giardia Lamblia an archezoan protoctist that lives in the human gut and causes a disease called giardiasis. (See p32, Fig 1.26)
A third example of paratism is a microsporidium Glugea sp. which is a parasite in the muscles of a fish causing a tumour. (See p59, Fig2.2)
Symbiosis has contributed greatly to diversification within protoctists, because of endosymbiotic origins of mitochondria and chloroplasts also through later interactions in which proctists acted as host cells or evolved into specialized parasitic or mutualistic symbionts.
Total words: 777
References:
Ridge, I. (2001) Ordering diversity, in Introduction to diversity, I. Ridge and C. M. Pond (eds), The Open University, Milton Keynes, pp29-35
Ridge, I. (2001) Diversity in protoctists, in Introduction to diversity, I. Ridge and C. M. Pond (eds), The Open University, Milton Keynes, pp55-63
Ridge, I. (2001) Diversity in protoctists, in Introduction to diversity, I. Ridge and C. M. Pond (eds), The Open University, Milton Keynes, pp85-87
Ridge, I. (2001) Diversity in protoctists, in Introduction to diversity, I. Ridge and C. M. Pond (eds), The Open University, Milton Keynes, pp90-92
