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Background

According to my A-level textbook: When Anopheles mosquitoes inject their proboscises and suck up blood from human host of malaria agent, they suck all stages of malaria parasite present in the blood like merozoites, cryptozoites, gametocytes etc. But, Anopheles mosquito's crop digests all other stages of malaria schizogony except the gametocytes.

Question

Well, how does the digestive system of Anopheles distinguish the gametocytes from other stages of malaria parasite? Or, is there any protective mechanism for gametocytes to stay untouched in face of digestion in Anopheles crop?

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Short Answer: Nothing prevents it, most Plasmodium (gametocytes/gametes/zygotes/ookinetes) are killed.

Background: After lots of search, I finally found a definite answer about this. According to Smith et al, 2014, the survival rate of Plasmodium in the Anopheles gut is very low. At each developmental step during their life cycle, a large proportion of Plasmodium face the deleterious effects of the gut (albeit they are not certain that the effect possesses contribution from enzymes). As an example, even if an Anopheles mosquito ingests thousands of Plasmodium gametocytes, about 50-100 are able to develop into ookinetes while only about five survive to form oocyst. This is why even in high transmission areas, majority of Anopheles mosquitoes are not infected by Plasmodium parasites. See this image:

Plasmodium bottlenecks

Pay attention to 'parasite #' and see how the number of parasites keeps on decreasing during different stages of development of Plasmodium.

Reference: Smith RC, Vega-Rodríguez J, Jacobs-Lorena M. The Plasmodium bottleneck: malaria parasite losses in the mosquito vector. Memórias do Instituto Oswaldo Cruz. 2014;109(5):644-661. doi:10.1590/0074-0276130597

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    $\begingroup$ Perhaps worth emphasising the points made in the paper (and shown in the Figure you have reproduced) that much of the reduction in parasite numbers may be attributable to factors present in the blood bolus such as complement, that come from the blood meal. $\endgroup$ – Alan Boyd Jul 13 '17 at 11:22
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    $\begingroup$ @Mockingbird yes, the paper that is linked to in the answer includes extensive discussion of the role of human complement in killing during the early stages - it is shown in the diagram along with blood-derived antibodies. The article also suggests a role for reactive nitrogen species generated by incoming haemoglobin, amongst other things. $\endgroup$ – Alan Boyd Jul 13 '17 at 11:42
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    $\begingroup$ @alanboyd the paper said that the gametocytes (inside bolus) are resistant to complememnts while ookinetes (which come out when complements and bolus are digested) are sensitive to complements. Enlighten me if I got this wrong, but this is why I didn't bother to add it. $\endgroup$ – another 'Homo sapien' Jul 13 '17 at 12:12
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    $\begingroup$ @mockingbird well I don't specialize in this field, so I can't say anything definitely $\endgroup$ – another 'Homo sapien' Jul 13 '17 at 12:13
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    $\begingroup$ @another'Homosapien' I thought that you had broadened the scope of the discussion beyond gametocytes. If you didn't mean to do that then fine, but looking at the information in the Figure I don't think it is fair to say that 'most of them are killed' if 'them' means gametocytes only. $\endgroup$ – Alan Boyd Jul 13 '17 at 12:32
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Disclaimer: I am trying to answer the question without finding any academic research on the topic. So, appreciate hypotheses and feel free to point out mistakes.

Short Answer : Co- Evolution


Broad Answer: 2 hypotheses

  1. The gamete stage of Malaria parasite simply co-evolved with Anopheles sp. to develop a protective mechanism to survive digestion in crop. Whereas, the other stages of Malaria parasite didn't co-evolve, so they get digested.

  2. Parasites had already the protective mechanism for surviving digestive enzymes in Anopheles crop. They entered mammals later and evolved a life cycle with a new protective mechanism for new host's immune system.

Why is that?

The answer to this question is not so clear according to known biology, as I haven't found much research in this phenomenon. But, we have to remember that biology sometimes (or always?) doesn't explain why rather explain how.

The following words are corresponding to what I think about the mechanism of this co-evolution

Hypothesis 1: It could have simply happened that some point in ancient times, the gamete entered the Anopheles crop from a mammal, but not only that; it tried to continue its life cycle on this new host. But, the environment of crop was unsuitable for the gamete to survive due tto digestive enzymes. So, the first few million or billion tries went into vain. But, the parasite slowly started to evolve to adopt in the Anopheles crop.

After 'some' time, the parasite developed a working protective mechanism from digestive enzymes and continued it's life cycle in Anopheles.

As @Another Home sapien points out, there should be some benefits for this evolution. While it's hard to tell exactly what the benefits could have been; I think it's simply to adopt to new potential host's physiological environment.

But why didn't other stages evolve of Malaria parasite in a same way?

This is another hard question to answer. However, it's not so that only the Anopheles and the parasite co-evolve; the mammal host co-evolved too in this period. And the whole system evolved gradually to get to the present life-cycle of malaria.

Hypothesis 2:

It might have happened that the original host to malaria parasite was indeed Anopheles. So, it had developed a protective mechanism long before entering any mammal host. Then, after entering the mammal host it developed another protective mechanism to survive host's immune system and continued it's life cycle.

I think hypothesis 2 is more probable as insects pre-existed mammals. Another hypothesis can be drawn the life cycle of parasite is shaped up simultaneously in mammal host and Anopheles host. But, it's hard to hypothesize what could have happened in this scenario.

Reference : 1. Wiki: Co-Evolution

2.Article on this co-evolution

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    $\begingroup$ I must say, you are assuming just too much in your answer. How do you know that Anopheles wasn't the original host of Plasmodium? And what benefit would sporozoites get even if they developed a protection against digestive enzymes? In evolution, selection pressure can be both in favor of or against a trait. Also, I won't recommend using phrases like "it chose to..." Also, usually its the parasite that co-evolves with the host, while the host just evolves to get rid of that parasite (which Anopheles too seems to be doing now)... $\endgroup$ – another 'Homo sapien' Jul 7 '17 at 15:50
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    $\begingroup$ @Mockingbird Just as an additional information: the definitive host of Plasmodium is the mosquito, not the mammal. $\endgroup$ – user24284 Jul 8 '17 at 6:00
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    $\begingroup$ Yes, totally. Do you know what a definitive host is? I used to warn my students to this detail all the time: the definitive host of Trypanosoma is the fly, humans are only secondary hosts; the definitive host of Anopheles is the mosquito, humans are only secondary hosts... maybe the fact that humans consider themselves so important has something to do with this common, yet incorrect, conception that we are the primary hosts for those parasites. $\endgroup$ – user24284 Jul 8 '17 at 6:18
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    $\begingroup$ @Mockingbird indeed a very imprtant & essential question to ask...@Gerardo Furtado said its totally correct.The Plasmodium's primary host is the Anopheles.There r reasons why but this logic is merely basic level only:The encysment of oocyst is due to partially secretion of cyst material by the stomach of the primary host and partially by the zygote itself.Without encystment sporogony doesnot initiate in them. This could be an important phenomenon which makes the mosquito not just a vector but a vector host.Otherwise they could hve cmpleted the entire cycle in humans alone. $\endgroup$ – user 33690 Jul 8 '17 at 9:01
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    $\begingroup$ @another'Homosapien' and Mockingbird: There is a reason why you cannot award the bounty to yourself (in any SE site). Suppose this situation: 1. You post a question and start a bounty of 500 (the maximum allowed). 2. People do a good research, excited about those 500 virtual whatever. 3. You get one or more well written, extensively documented answer. 4. You post your own, crappy answer. 5. You benefit from the canonical answer but award yourself the bounty, not loosing any point. $\endgroup$ – user24284 Jul 13 '17 at 13:41

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