Introduction

Insects are one of the prime sources of karyological research. An entomologist tries to study each and every aspect of insects, but the most negligible and critical aspect is the chromosomal studies. Chromosomal analysis provides information about the genetic structure and nature of an organism and displays a wide range of variations. Cytological studies contribute in three ways: (1) to design natural and phylogenetic relationship among the various groups (2) to understand various cytogenetic processes in the evolution of different groups and (3) to solve twitched cases like variation among geographical races, individual abnormalities and polymorphic morphs.

Order Odonata includes three suborders, Zygoptera (damselflies), Anisoptera (dragonflies) and Anisozygoptera. Suborder Zygoptera comprises of 3162 species under 319 genera globally and 211 species under 59 genera and 9 families are present in India. Family Coenagrionidae is considered as one of the largest damselflies family. Taxonomically, 1351 species and 121 genera are present all over the world and 60 species and 12 genera are recorded in India ¹. The first coenagrionid species, Ceriagrion rubiae was studied by Asana and Makino ².  They documented that an unpaired X chromosome migrated to one pole during ³ secondary spermatocyte division in all the species. Size of m chromosomes varies from species to species and is closely related to the size of X element. However, in Ceriagrion rubiae, size of m chromosome is found to be equal to X chromosome. Kuznetsova and Golub revise the checklist of chromosome numbers for Odonata, which covers 92 species of the family Coenagrionidae. Presently, a review on 107 species of family Coenagrionidae has been catalogued and discussed based on key cytogenetic characteristics of the family (Table I). For this, the following parameters have been undertaken.

Holokinetic chromosomes

Evolution of chromosome number

m chromosomes

Recombination index

Sex determining mechanism

Table 1: Worldwide List of Cytogentically Studied Species of the Family Coenagrionidae.

Holokinetic Chromosomes

One of the fundamental components of chromosome structure is its kinetic organization. This has always been a disputed topic in the case of odonates. Oksala ^{4, 5, 6, 7} expresses that odonates possess localized centromere and his view has been shared by Dasgupta ⁸, Seshachar and Bagga ⁹ and White ¹⁰. On the other hand, Piza ^{11, 12} reports dicentric chromosomes in odonates, while Schrader ¹³, Hughes- Schrader¹⁴ and Lima de Faria¹⁵ agree with the opinion of diffused kinetochores.

In presently studied 21 Indian species it is found that two parallel chromatids without any constriction in chromosomes are seen during the spermatogonial metaphases of Ceriagrion cerinorubellum, Ceriagrion coromandelianum, Ischnura elegans, Ischnura senegalensis and rod shaped chromosomes are present in Aciagrion hisopa, Agriocnemis pygmaea, Ischnura aurora, Ischnura forcipata. At the time of late diakinesis, bivalents of holocentric chromosomes appear to be held together by end to end association due to the terminalisation of chiasmata. This is seen in almost all the studied species, while chromatids are seen to separate by parallel disjunction during anaphase- I in Agriocnemis pygmaea. The autosomal bivalents appear to be rod shaped in metaphase- I in all the studied species and when it enters in metaphase- II, the size of the chromosomes remain half as seen in Ischnura aurora, Ischnura forcipata, Pseudagrion laidlawi and Pseudagrion rubriceps. This type of chromosome behaviour also supports the holokinetic chromosomes in Odonata.

Evolution of chromosome number

Kiauta^{16, 17, 18} finds haploid number 12, 13 and 14 to be present in more than 90% of Odonata. He considers n=13 to be the type number of the order, which has been cytologically reported in 58% of studied species. Numerical variation in Odonata karyotype due to occurrence of breaks (leading to haploid numbers 10-15) and fusions (leading to haploid complements 3-7) has been explained graphically by him (Fig. 1). He combines genealogical observations of Fraser¹⁹ with cytological findings and concluded that family Coenagrionidae, Aeshnidae and Libellulidae are the most advanced and dominant families, which are of independent origin and are more ancient than present day dragonflies. He also considers greater chromosome numbers as an indication of advancement of families. Further, Kiauta ²⁰ refers that in suborder Zygoptera only 39.4% possess n=13 and 53.6% show n=14. However, n=14 is peculiar only to the families Coenagrionidae and Protoneuridae. So, he considers n=13 as the type number of suborder Zygoptera. In the family Coenagrionidae, majority of the species possess type number n=14m, while variations (25% of species) in chromosome complement due to fusions (in 10% of species) and fragmentations (in 13 % of species)  have been also reported (Table-1).

Figure 1: Proposed hypothesis of karyotypic evolution in Odonata by Kiauta (1967c). Click here to view figure

Fragmentations have been found in Argia apicalis ²¹ and Argia tibialis ²² (n=19); in  Ceriagrion cerinorubellum ²³, Enallagma cyathigerum ²⁴, Ischnura inarmata ²⁵, Ischnura pumilio ²⁵ and Leptagrion macrurum ²⁶ (n=15m); in Ceriagrion coromandelianum ²⁷ (n=21/23) and in Pyrrhosoma nymphula ⁵ (n=28). Sandhu and Walia ²⁸, Walia and Sandhu  ²⁹ and Walia ³⁰ report aberrant autosomal fragmentations in some species and conclude that aberrant fragmentations occur due to the effect of pollutants to increase the recombination index, which favour the flexibility of the genotype for adaptation of the species in the polluted water [Ceriagrion coromandelianum (2n♂=27/41/45), Coenagrion dyeri (2n♀ =28, 36, 56, 58), Pseudagrion rubriceps (2n♂,♀= 27, 37, 43, 45), Pseudagrion decorum (2n♂= 27, 34, 42, 52, 56, 58) and Agriocnemis obscura (2n♀= 28, 36, 44, 52, 56)].

Autosomal fusions in the family Coenagrionidae have been reported in Mecistogaster sp. ³¹ 2 (n=6) in Agriocnemis pygmaea ³² (n=12); in Coenagrion mercuriale mercurial ^{33, 34} and Pseudagrion whellan ³⁵ (n=13). Reduction in chromosome number (n=13) due to the absence of m chromosomes has been found in Ischnura aurora ^{27, 37} and Ischnura forcipata ³⁸.

m chromosomes

m chromosomes have been used as a standard in comparative karyotypic studies of dragonflies for the first time by Ray Chaudhuri and Dasgupta ³⁹. Kiauta ⁴⁰ considers m chromosomes as fragments of normal autosomes, which occur by the fragmentations in normal chromosomes. Out of total 107 cytogenetically studied coenagrionid species, m chromosomes are found to be present in 35 species, absent in 50 species, while in 22 species m chromosomes show variations (means absent and present) (Table I). Absence or presence of m chromosome in the species might be due to the geographical isolated populations of the species. In present study, species n=14 (without m chromosomes) has been reported in Amphiallagma parvum (Himachal Pradesh, India), while n=14m observed in the same species (as named- Enallagma parvum) by Handa and Kochhar ⁴¹; Walia ^38; Walia and Sandhu ²⁷ (Punjab, India).

Recombination Index

Recombination index is the sum of number of bivalents and average number of chiasmata per nucleus can be considered key character of genetic system of a species (Darlington, 1939). Chiasma frequency is fixed in the order Odonata and change in chromosome number is the only way of changing the recombination index. Chromosomal fusions and fragmentations play a major role in the evolution and phylogeny of the order ^{42, 25, 35, 43, 44, 45, 28, 46, 47, 29, 48}.

In the family Coenagrionidae, most of the male damselflies possess single chiasma per bivalent which indicates that the species are well adapted to its habitat. Increase in recombination index by autosomal fragmentation has been reported in Enallagma cyathigerum (n= 15m) ^{24, 49}, Ceriagrion cerinorubellum (n= 15m) ⁵⁰, Ischnura pumilio (2n=29) ⁵¹, and Ceriagrion coromandelianum (n=14m/21/23) ²⁷. Moreover, aberrant fragmentations due to the effect of pollutants have been found in Ceriagrion coromandelianum (2n♂==27, 37, 45), Coenagrion dyeri (2n♀=28, 36, 56, 58), Pseudagrion rubriceps (2n♂=27, 37, 43, 45 and 2n♀= 28, 41, 45), Pseudagrion decorum (2n=27, 34, 42, 52, 54) and Agriocnemis obscura (2n♀=28, 36, 44, 52, 56) ^{28, 46, 29, 27, 30}.   

Autosomal fusions decrease the chromosome number as well as recombination index, which favour the survival value and reproductive capability of the species to settle over the whole geographical range for ecological adaptation. In the family Coenagrionidae, autosomal fusions have been reported in Pseudoagrion whellani (n=13) ²⁵ and Agriocnemis pygmaea (n=12) ³² and Coenagrion mercuriale mercuriale (n=13) ⁴⁵.

It has been reported that in the currently studied species, 17 coenagrionid species only have one chiasma per bivalent, while Agriocnemis femina and Ischnura rufostigma have two large autosomal bivalents, and Pseudagrion laidlawi, Pseudagrion microcephalum and Pseudagrion rubriceps have one large autosomal bivalent that has both interstitial as well as terminal chiasma. Increase in number of chiasmata increases the recombination index of the species, which promote flexibility of the genotype for adaptation and considered as cytological marker of the species.

Sex determining mechanism

All the primitive orders of exopterygote insects seem to have male heterogamety, usually of XO type. Numerous instances are seen in which XO: XX sex chromosomes systems reverted to XY: XX form in case of acrocentric chromosomes. Such a fusion is said to create a neo- X chromosome and when the fusion reaches the fixation in the population, the original acrocentric autosome is confined to the male line and constitutes neo- Y ¹⁰. Kiauta ⁵⁰ also considers XO: XX sex determining mechanism as the most primitive condition of the odonates and presented the graphical interpretation of the evolution of sex determining mechanisms in dragonflies (Fig. 2). In the family Coenagronidae, majority of the species possess XO-XX type of sex determining mechanism. However, neo- XY mechanism has been reported in Leptagrion macrurum ²⁶, in Agriocnemis pygmaea ³² and in Mecistogaster Sp. 2 ³¹. According to reports, the sex chromosome in males is univalent achiasmatic and exhibits bipartite behaviour during meiosis I, supporting the stability of the XO-XX type of sex determination in the Coenagrionidae family.

Figure 2: Evolution of the sex determining mechanisms in the order (Kiauta, 1975). Click here to view figure

Conclution

Chromosome number of coenagrionid species varies over a relatively wide range, from n = 6 in Mecistogaster sp. 2 ³¹ to n = 38 in Pseudagrion decorum ²⁷, while n=14m is considered as the type number of the family Coenagrionidae. Increase in chromosome number is related to evolutionary advancement of families, Coenagrionidae and Libellulidae, while decreased in number is related to primitive families, Chlorocyphidae and Gomphidae. Decrease in chromosome number also occurs due to the gradual diminution and ultimate disappearance of the m chromosomes. The presence of contaminants in the stagnant water causes an aberration in the chromosomal complement of the species.

Acknowledgements

We acknowledge technical support of Department of Zoology and Environment Sciences and Sophisticated Instruments Centre, Punjabi University, Patiala. Punjab, India.

Conflicts of Interest

Authors declare no competing interests.

Funding Sources

This work was supported by CSIR, NEW DELHI, [grant 37(1716)/18/EMR-II] to Gagandeep Kaur Dhillon (SRF) in the CSIR Project entitled “DNA Barcoding of Dragonflies and Damselflies (Odonata: Insecta) based on Mitochondrial COI Gene” under the supervision of Dr. Gurinder Kaur Walia (Principal Investigator), Department of Zoology and Environmental Sciences, Punjabi University, Patiala.

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