![Central fusion and terminal fusion automixis](https://upload.wikimedia.org/wikipedia/commons/e/ed/Central_fusion_and_terminal_fusion_automixis.svg)
''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
Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and development ...
.
Diploidy might be restored by the doubling of the chromosomes without cell division before meiosis begins or after meiosis is completed. This is referred to as an ''
endomitotic'' cycle. This may also happen by the fusion of the first two
blastomeres
In biology, a blastomere is a type of Cell (biology), cell produced by cleavage (embryo), cell division (cleavage) of the zygote after fertilization; blastomeres are an essential part of blastula formation, and blastocyst formation in mammals.
Hum ...
. Other species restore their ploidy by the fusion of the meiotic products. The chromosomes may not separate at one of the two anaphases (called ''restitutional meiosis'') or the nuclei produced may fuse or one of the polar bodies may fuse with the egg cell at some stage during its maturation.
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. Among these authors, the threshold for classifying automixis as a sexual process depends on when the products of anaphase I or of anaphase II are joined together. The criterion for "sexuality" varies from all cases of restitutional meiosis,
to those where the nuclei fuse or to only those where gametes are mature at the time of fusion.
Those cases of automixis that are classified as sexual reproduction are compared to
self-fertilization
Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species o ...
in their mechanism and consequences.
The genetic composition of the offspring depends on what type of apomixis takes place. When endomitosis occurs before meiosis
[Cosín, Darío J. Díaz, Marta Novo, and Rosa Fernández. "Reproduction of Earthworms: Sexual Selection and Parthenogenesis." In Biology of Earthworms, edited by Ayten Karaca, 24:69–86. Berlin, Heidelberg: Springer Berlin Heidelberg, 2011. https://doi.org/10.1007%2F978-3-642-14636-7_5.] or when ''central fusion'' occurs (restitutional meiosis of anaphase I or the fusion of its products), the offspring get all
to more than half of the mother's genetic material and heterozygosity is mostly preserved
(if the mother has two alleles for a locus, it is likely that the offspring will get both). This is because in
anaphase I
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately res ...
the homologous chromosomes are separated. Heterozygosity is not completely preserved when crossing over occurs in central fusion.
In the case of pre-meiotic doubling, recombination -if it happens- occurs between identical sister chromatids.
If ''terminal fusion'' (restitutional meiosis of anaphase II or the fusion of its products) occurs, a little over half the mother's genetic material is present in the offspring and the offspring are mostly homozygous.
This is because at anaphase II the
sister chromatids
A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the dup ...
are separated and whatever heterozygosity is present is due to crossing over. In the case of endomitosis after meiosis, the offspring is completely homozygous and has only half the mother's genetic material.
This can result in parthenogenetic offspring being unique from each other and from their mother.
References
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Asexual reproduction
Zoology