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I have seen that in some books refer to Parthenogenesis as sexual and in others as asexual. The ones that say it's a form of sexual reproduction,they focus on the fact that meios has happened in the process and also,we are using a sexual reproduction system in that animal to produce the offspring. The ones saying it is a from of asexual reproduction focus on the fact that although meios is involved,we only have one parent in the reproduction process and we cannot have any new alleles in the offspring than the ones the parent has.(unless we have mutation)

so how can we conclude in the end?which is it ?

and also,what is the evolutionary explanation for this phenomenon(parthenogenesis)?

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  • $\begingroup$ Could you add some definition of meiosis? Does meiosis imply crossing over of genes in the context of your question? $\endgroup$ Mar 3 at 14:34

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Only a matter of definition

It is only a matter of definition. When it comes to sex, there is such a diversity of mating system that it is very hard to make a good definition that won't be challenged by limit cases.

Sexual reproduction can typically be defined based on the presence of meiosis (but again this will lead to limit-cases), on the need for two distinct parents + meiosis, or on the need for two distinct parents which have different gender (anisogamy) + meiosis.

Diversity of reproductive systems

At the end of the day, what one wish to call sexual reproduction does not matter too much as long as the definition is being made clear especially when dealing with potentially limit-cases.

If I remember correctly the intro book to evolutionary biolgy called Evolution by Futuyma makes a good overview of the diversity of reproductive system. Having a vague understanding of this diversity will help you undermine your hope of finding a clearly defined definition for sexual reproduction.

Applying the definition of wikipedia to parthenogenesis

Here is the definition from wikipedia

Sexual reproduction is a form of reproduction where two morphologically distinct types of specialized reproductive cells called gametes fuse together, involving a female's large ovum (or egg) and a male's smaller sperm

Using this definition and applying it to the specific case of parthenogenesis, one would conclude that parthenogenesis is not a form of sexual reproduction (despite the presence of meiosis). Note by the way the even within the term parthenogenesis, there is quite a bit of diversity (see the wikipedia entry for parthenogenesis).

what is the evolutionary explanation for this phenomenon(parthenogenesis)?

I will ignore this question because

  1. A post should always be limited to a single question
  2. This question is very broad
  3. There is not a single answer to this question given the diversity of types of parthenogenesis.

For a start on the subject, you might want to have a look at Jaenike and Selander, 1979 for example.

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  • $\begingroup$ Thank you very much for your answer.I was still wondering,How can one decide which is more important in the definition ? the presence of Meiosis or the presence of the two parents? $\endgroup$
    – Tarannom
    Nov 30, 2016 at 17:37
  • $\begingroup$ More important for what? Also, it is not that easy to really decoupled the two as as soon as you have two parents, you need some mechanism to merge their genetic information (mechanism which is meiosis). If you are asking "is segregation more important than recombination?", then one first needs to acknowledge that segregation is nothing but a recombination rate of 0.5 at the boundaries between chromosomes (more info about recombination rate and their statistics here) $\endgroup$
    – Remi.b
    Nov 30, 2016 at 18:03
  • $\begingroup$ "... on the need for two distinct parents + meiosis, or on the need for two distinct parents which have different gender (anisogamy) + meiosis..." - Could you add some reference or name some example for two distinct parents that are supposed not to have different gender (no anisogamy)? Some funghi? $\endgroup$ Mar 3 at 14:38
  • $\begingroup$ "... more important in the definition ? the presence of Meiosis or the presence of the two parents?" makes we want to set up some answer, are you still interested in the subject? Interestingly, two "parents" is not what the commented answer assumes when citing the textbook definition: it's two morphological different gametes! They might be derived from one single "tree" (blossom male und female, same tree, no two "parents"). Again, very pertaining to differentiate two parents and/or gametes from crossing over of allelic genes. Please let me know if you need any adding answer. $\endgroup$ Mar 3 at 14:51
  • $\begingroup$ "applying wikipedia's definition...": "zygote that develops into an organism composed of cells with two sets of chromosomes (diploid)" That should also be addressed, if gametes can be derived from one single cells or, if not, from its mitotic clone without those cells forming "an organism composed of cells", full stop. $\endgroup$ Mar 3 at 15:06
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Wikipedia on Automixis says:

"Some authors consider all forms of automixis sexual as they involve recombination. Many others classify the endomitotic variants as asexual and consider the resulting embryos parthenogenetic."

From the last words of this difficult statement it may reversely be infered that parthenogenesis is defined as non-sexual. There do exist different definitions of "sexual reproduction". What some authors consider non-sexual and parthenogenetic, others consider as sexual and: non-parthenogenetic. All agree on the term parthenogenetic denoting a-sexual reproduction.

However, it also results from Wikipedia that in the context of parthenogenesis, the existence of haploid gametes, contrary to common definition, is not a prerequisite of sexual reproduction, as with all forms of automixis haploid gametes are not involved.

As the question seems to assume there are different opinions on some forms of automixis being sexual, thus parthenogenetic, or not. To easen up: you must look at the action - what looks like parthenogenesis maybe is not sexual, opinion diverge

Automixis may loosely be used as a synonym for parthenogenesis process and vice versa, the more if parthenogenesis is seen as the opposite of any mating form of reproduction that uses gametes (or is hermaphroditic). As far as some authors consider automixis that mitotically produces a stage of haploid cells as sexual reproduction, thus non-parthenogenetic they are looked upon by other authors who do not share that definition as speaking of "sexual parthenogenesis". Some kind of rebuff, I think, as no author defines parthenogenesis as sexual.

Some say that forms of automixis that involve some kind of - haploid - "gametes" (think of haploid cells as the first stage of a developement of gametes) should not be considered parthenogenesis, but sexual reproduction.

Conversely, automixis and the crossing-over of genes is not a sufficent condition for defining those processes as sexual, thus non-parthenogenetic. All agree that "mixis" alone is no reason to categorize the specific process as non-parthenogenetic.

To easen up: Standard, normal, processes of parthenogenesis do not imply the recombination of genomes, and are termed non-sexual without discussion (which might come to a surprise and is nothing trivial either, and that might have been one reason to put the question).

Quote from your question: "The ones that say (parthenogenesis is) a form of sexual reproduction,they focus on the fact that meios has happened in the process..." That denotes the very point where different opinions come together. To easen up: The ones that say so do not intend to define those "meiotic"/crossing over forms of reproduction as parthenogenic. On the contrary (this is intricate, sorry for being clumsy or "not learned") they want those forms to be counted out. Counter-intuitively, those authors do not see those processes as parthenogenic (without considering them as hermaaphroditic). So there is sexual reproduction with no gametes as cells that have had some life, different lives. As it occurs only now: those who consider the production of "tiny" haploid cells as sufficient to render the process sexual in charakter, their point of view can easily be reconciled with a more general, common, definition of sexual reproduction that is based on the use of gametes; the concept of gametes is expanded on those haploid "endomitotic" cells that instantenously fuse and are derived from one and the same individual (as it may occur now: the character is hermaphroditic, then!)

You may find that irritating frame shift in the very first sentence of Wikipedia (to my mind the following quote might have been incited your question):

Wikipedia: "Automixis[...] is the fusion of (typically haploid) nuclei or gametes derived from the same individual.[...] The term covers several reproductive mechanisms, some of which are parthenogenetic." They should not say "are" (parthenogenetic).

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