Nuclear dualism (dualism from lat. Dualis - dual) is the heterogeneity of the nuclei of the same cell, characteristic of a number of groups of protists . I. B. Raikov defines nuclear dualism as the simultaneous structural differentiation of the nuclei of one cell. With this definition, cases of genetic heterogeneity and functional differentiation of nuclei that do not affect their morphology are excluded from this phenomenon. Typical nuclear dualism is found in ciliates and many representatives of foraminifera . As a rule, it is expressed in the presence of two types of nuclei - somatic (in ciliates they are called macronucleus , Ma) and generative (in ciliates - micronucleus , Mi). Active transcription occurs in somatic nuclei, so that their genome directs the synthesis of proteins in the cell and determines its phenotypic characters. Generative nuclei are usually transcriptionally inactive; they ensure the transfer of genetic information to future generations during the sexual process.
Nuclear dualism in ciliates
As a rule, Mi ciliates are transcriptionally inactive, and Ma is highly active. Nevertheless, there are species and strains of ciliates that die without Mi within a few days [1] . Perhaps their genes express individual genes. Other species and strains (both obtained artificially and found in nature) can exist without Mi for hundreds of generations.
As a rule, Ma are larger and often complex in shape. For example. in spirostomum ( Spirostomum ), they are macaque ; in blepharism ( Blepharisma ), they are ribbon-like. Most ciliates have only Ma, but often Ma (like Mi) is present in a large number in the cell. So, in dileptus ( Dileptus ) up to 500 Ma.
Mi ciliates are usually diploid, Ma is usually polyploid. The only group that has “diploid” Ma is a detachment of karyorelictids. However, although the DNA content in Ma is approximately the same as in Mi, some of the sequences are eliminated during the development of Ma. and the rest are replicated [2] . Thus, in this case, the genomes of Ma and Mi are different.
Foraminifera Nuclear Dualism
Nuclear dualism has been found in the agamonts of some multicamera foraminifers . As a rule, in their cells there are several diploid generative nuclei and one somatic nucleus, but some forms of somatic nuclei may have several. The somatic nucleus is distinguished by its larger size and the presence of a nucleolus or several nucleoli; generative nuclei remain small, with compact chromatin packaging. As the number of chambers increases, the somatic nucleus usually moves to younger chambers, and the generative nuclei remain in the original chamber.
Somatic nuclei in some foraminifera are diploid, and in others polyploid.
During reproduction, generative nuclei undergo meiosis, and the somatic nucleus always degenerates. If a somatic nucleus is destroyed or removed by microsurgery in a cell, one of the generative nuclei turns into a somatic nucleus. This suggests that there is a mechanism that impedes the transformation of generative nuclei into somatic ones in the presence of a functioning somatic nucleus in the clect.
See also
- Heterocaryon
Literature
- Raikov I. B. The core of protozoa. Morphology and evolution. L., Science, 1978
- Lukashenko N.P., Rybakova Z. I. Structure and function of protozoa genomes. M., Science, 1991