Review Article |
Corresponding author: Vladimir E. Gokhman ( vegokhman@hotmail.com ) Academic editor: Danon Clemes Cardoso
© 2020 Vladimir E. Gokhman.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Gokhman VE (2020) Chromosomes of parasitic wasps of the superfamily Chalcidoidea (Hymenoptera): An overview. Comparative Cytogenetics 14(3): 399-416. https://doi.org/10.3897/CompCytogen.v14i3.56535
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An overview of the current knowledge of chromosome sets of the parasitoid superfamily Chalcidoidea is given. Karyotypes of approximately 240 members of this group, i.e. just above one percent of described species, are studied up to now. Techniques for obtaining and analyzing preparations of chalcid chromosomes are outlined, including the so-called “traditional” and “modern” methods of differential staining as well as fluorescence in situ hybridization (FISH). Among the Chalcidoidea, the haploid chromosome number can vary from n = 3 to n = 11, with a clear mode at n = 6 and a second local maximum at n = 10. In this group, most chromosomes are either metacentric or submetacentric, but acrocentrics and/or subtelocentrics also can predominate, especially within karyotypes of certain Chalcidoidea with higher chromosome numbers. The following main types of chromosomal mutations are characteristic of chalcid karyotypes: inversions, fusions, translocations, polyploidy, aneuploidy and B chromosome variation. Although karyotype evolution of this superfamily was mainly studied using phylogenetic reconstructions based on morphological and/or molecular characters, chromosomal synapomorphies of certain groups were also revealed. Taxonomic implications of karyotypic features of the Chalcidoidea are apparently the most important at the species level, especially among cryptic taxa.
base-specific fluorochromes, chalcid wasps, differential staining, FISH, karyotypes, phylogeny, taxonomy
The superfamily Chalcidoidea is a very diverse, taxonomically complicated and economically important group of insects (
Perhaps it is needless to mention that tissues with relatively large numbers of cell divisions should be examined to perform a successful chromosomal analysis of any given group. In the case of Hymenoptera, this for a long time meant studying immature stages (
Nowadays, the technique developed by
To visualize chromosomes of Chalcidoidea, modern optic microscopes are currently used. Additional epifluorescence modules are also needed to work with fluorochromes, including base-specific chromosome staining and FISH. Moreover, the resulting images must be captured by a modern digital camera, usually controlled through a computer. This camera should produce images with relatively high resolution (at least 300 dpi) and be sensitive enough to work with fluorescence. In turn, these images can be analyzed using specialized software, e.g. KaryoType (
It is also noteworthy that precise species identifications are crucial for the karyotypic study of Chalcidoidea as well as of parasitoid Hymenoptera in general (
Karyotypes of the overwhelming majority of chalcids were studied using only routine staining. Nowadays, chromosomes of Chalcidoidea are most often stained with Giemsa solution diluted in Sorensen’s phosphate buffer (
In addition, karyotypes of a few dozen members of the superfamily Chalcidoidea were examined using various methods of differential staining (
G-banding is usually produced by treatment of chromosomes with certain proteolytic enzymes like trypsin (
The modern techniques of differential chromosome staining are mostly represented by using fluorochromes which specifically visualize AT- and GC-rich chromosome segments (
Nevertheless, FISH remains the most powerful tool for analyzing chromosomes of parasitoid Hymenoptera including chalcids (
Methods of immunocytochemistry also can be used for studying karyotypes of parasitoid Hymenoptera. Up to now, however, this technique was applied only to two closely related species, Entedon cioni Thomson, 1878 and E. cionobius Thomson, 1878 (Eulophidae) (
In the superfamily Chalcidoidea, haploid chromosome numbers (n) can vary from n = 3 to n = 11 (Table
Distribution of main lineages of Chalcidoidea by the chromosome number at the species level (based on data from Table
Just a decade ago (
Representative karyotypes of Chalcidoidea a Trichogramma principium Sugonjaev & Sorokina, 1976 (Trichogrammatidae; n = 5) b Mesopolobus mediterraneus (Mayr, 1903) (Pteromalidae; 2n = 10) c Oomyzus gallerucae (Fonscolombe, 1832) (Eulophidae; 2n = 12) d Eurytoma cynipsea Boheman, 1836 (Eurytomidae; 2n = 20 + 4B). Scale bar: 10 µm.
Variation ranges of chromosome numbers of Chalcidoidea mapped on the phylogenetic tree of chalcid families (simplified from
Chromosome numbers of different families of Chalcidoidea. Spalangiinae were earlier considered as a subfamily of Pteromalidae s.l., but they deserve the family rank (
Family | No. species studied | Chromosome numbers (n) |
---|---|---|
Mymaridae | 3 | 9, 11 |
Eulophidae | 73 | 5, 6, 7, 8, 10 |
Trichogrammatidae | 11 | 5 |
Aphelinidae | 31 | 3, 4, 5, 6, 7, 8, 9, 10, 11 |
Agaonidae | 8 | 5, 6 |
Encyrtidae | 20 | 5, 8, 9, 10, 11 |
Eupelmidae | 22 | 5, 6, 7, 8, 10 |
Eurytomidae | 14 | 5, 6, 7, 8, 9, 10 |
Spalangiinae | 2 | 4, 6 |
Leucospidae | 1 | 6 |
Chalcididae | 5 | 3, 5, 6 |
Ormyridae | 2 | 5, 6 |
Torymidae s.l. | 24 | 4, 5, 6, 10 |
Perilampidae | 1 | 3 |
Eucharitidae | 1 | 4 |
Pteromalidae | 19 | 4, 5, 6, 7 |
Total | 237 | 3, 4, 5, 6, 7, 8, 9, 10, 11 |
Chromosomes of Chalcidoidea are generally longer than those found in many other parasitoid Hymenoptera, mainly due to lower chromosome numbers that are characteristic of most chalcids, with average chromosome lengths ranging from 5 to 7 μm (
Among Chalcidoidea, meiotic chromosomes were examined in some detail in a few dozen members of the families Eulophidae, Aphelinidae, Encyrtidae, Eupelmidae, Eurytomidae, Torymidae s.l. (including Megastigmidae) and Pteromalidae (
The following types of chromosomal mutations are characteristic of chalcid karyotypes: (
At present, direct evidence for translocations, which occur among Chalcidoidea, is generally scarce. For instance, reciprocal translocations are presumed in certain members of the family Eulophidae (
Fortunately, other types of chromosomal mutations can be identified more easily among the Chalcidoidea, because these karyotypic changes usually affect the chromosome number of related forms. For example, this parameter decreases via chromosomal fusions, and the products of these rearrangements can be instantly detected using e.g. chromosome morphometrics or whole chromosome painting (
Polyploid individuals were found in a few groups of Chalcidoidea. For example, triploid females were found in Nasonia vitripennis and certain Aphelinidae (
At present, the only reliable case of aneuploidy among chalcids is known in Torymus bedeguaris (Linnaeus, 1758) (Torymidae). In this species, which usually has 2n = 12, three copies of the smallest acrocentric chromosome carrying NORs were found in the only specimen with 2n = 13 (
Up to now, B chromosomes were found in certain members of the superfamily Chalcidoidea. Specifically, the so-called PSR (paternal sex ratio) B chromosomes were detected in two distantly related chalcid species, i.e. Nasonia vitripennis and Trichogramma kaykai (
Chalcid karyotype evolution was previously studied using phylogenetic reconstructions that were based on morphological and/or molecular characters (
The problem of phylogenetic reconstruction of karyotype evolution at the level of higher taxa can be illustrated by the example of the Eulophidae, apparently the best studied group of the superfamily Chalcidoidea (Table
In addition, numerous chromosomal fusions lead to independent origins of similar karyotypes within different lineages of Chalcidoidea (
In the superfamily Chalcidoidea, karyotypic features can have substantial taxonomic implications, and these implications are the most important at the species level (
Variation of chromosome morphology between routinely stained karyotypes of related species with the same n values was also revealed. For instance, two reproductively isolated populations of Encarsia sophia (Girault & Dodd, 1915) (Aphelinidae) from Spain and Pakistan have structurally different karyotypes with n = 5 (
In the coming decades, karyotypic study is undoubtedly going to become an important tool of taxonomic and cytogenetic research on many groups of parasitic wasps, including chalcids. However, this investigation can be effective only if complemented by other modern approaches and techniques. For example, it should be used in combination with a thorough morphological analysis for detecting and identifying cryptic species of parasitoids (
Although a considerable amount of new data of the karyotypic study of the superfamily Chalcidoidea were collected and summarized during the last decade (see e.g.
The author is grateful to many Russian and foreign colleagues for providing living material for the karyotypic study and for identifying specimens of Chalcidoidea as well as for the useful discussion. The present work was partly supported by a research grant no. 18-04-00611 from the Russian Foundation for Basic Research.