Cytogenetic studies in Pentatomidae (Heteroptera): A review

Introduction

The suborder Heteroptera constitutes one of the most important insect groups because most of its species are plant feeders during both nymphal and adult stages, and cause great damage on many plants of economic importance. With approximately 37 000 nominal species the suborder comprises eight infraorders, five of which include agronomically important species (Rizzo 1976; Saini 1992; Schuh and Slater 1995; Schaefer and Panizzi 2000; Triplehorn and Johnson 2005).

The superfamily Pentatomoidea includes 1080 genera and 5907 species belonging to 16 families. Cydnidae, Pentatomidae, Scutelleridae and Tessaratomidae are the most important since 94% of the species belong to them (Schaefer 1993; Schuh and Slater 1995; Schaefer and Panizzi 2000).

One of the largest families within Heteroptera is Pentatomidae including 4112 species (Schaefer and Panizzi 2000). They are called stinkbugs because they produce a disagreeable odour by means of scent glands that open in the region of the metapleuras. Schuh and Slater (1995) include eight subfamilies in this family: Asopinae, Cyrtocorinae, Discocephalinae, Edessinae, Pentatominae, Phyllocephalinae, Podopinae and Serbaninae. Packauskas and Schaefer (1998) treated Cyrtocoridae as a family, and J. Grazia (personal communication) removed Serbaninae of Pentatomidae. Most economically important phytophagous species belong to the subfamilies Edessinae and Pentatominae, and the latter contains the majority of species that are crop pests. The species belonging to Asopinae are predators, and some of them are important agents of biological control (Schaefer and Panizzi 2000).

Until present more than 1200 heteropteran species belonging to 42 families have been cytogenetically analysed (Ueshima 1979; Manna 1984; Schuh and Slater 1995). The diploid chromosome number ranges from four (Lethocerus sp., Belostomatidae) to 80 (four species of Lopidea Uhler, 1872, Miridae). Cytogenetic reports on Pentatomoidea refer to 391 species belonging to only nine families: Acanthosomatidae (12 species), Cydnidae (14 species), Dinidoridae (12 species), Plataspididae (16 species), Tessaratomidae (nine species), Thaumastellidae (two species), Scutelleridae (27 species), Urostylididae (five species), and Pentatomidae, which is by far the more studied (294 species) (Ueshima 1979; Manna and Deb-Mallick 1981; Nuamah 1982; Manna 1984; Dey and Wangdi 1988; Satapathy and Patnaik 1988, 1989; Jacobs 1989; Satapathy et al. 1990; González García et al. 1996; Rebagliati 2000; Rebagliati et al. 2001, 2002, 2003; Lanzone et al. 2003; Kerzhner et al. 2004).

One of the most important cytogenetic characteristics of Heteroptera is the holokinetic nature of its chromosomes, i.e. the presence of a diffuse kinetic activity, which contrasts with the monocentric nature (i.e. with a localized centromere) of the chromosomes of most organisms. Holokinetic chromosomes have been reported in other insect groups such as the so-called Homoptera, Phthiraptera, Lepidoptera, Odonata, Trichoptera, Psocoptera and Zoraptera (Hughes-Schrader and Schrader 1948; Blackman 1985; Mola 1995; Traut and Marec 1997; Tombesi et al. 1999; Kuztnezova et al. 2002), in some arachnids (Shanahan 1989; Rodríguez Gil et al. 2002) and in nematodes (Goday et al. 1992; Dernburg 2001). All organisms with holokinetic chromosomes show the same mitotic behaviour: chromosomes orientate at the metaphase plate with their long axes perpendicular to the spindle axis and sister chromatids migrate parallel to each other. However, the meiotic behaviour presents differences according to the group of organisms, since bivalents can divide pre- or post-reductionally (Battaglia and Boyes 1955; White 1973). In holokinetic systems, fusions and fragmentations are the most frequent chromosome rearrangements that cause variations in diploid number.

The male sex chromosomes described so far in Heteroptera are XY (approximately 75% of the species), XO, different multiple systems X_nO, X_nY and XY_n, and less frequently neo-XY (in only five species) (Ueshima 1979; Manna 1984; Grozeva and Nokkala 1996; Grozeva 1997; Nokkala and Grozeva 1997; Bressa et al. 1999; Nokkala and Nokkala 1999).

Two hypotheses concerning the evolution of the sex determining systems have been proposed in Heteroptera. Ueshima (1979) suggested that the XY system derived from an ancestral X0 which is commonly encountered in primitive heteropteran taxa and in primitive insect orders as well. By contrast, Nokkala and Nokkala (1983) suggested that the X0 system derived from the XY, present in the majority of the species analysed, on the basis of the discovery of an Y chromosome in Saldidae. New evidences of primitive taxa support the latter suggestion (Nokkala and Nokkala 1984; Grozeva and Nokkala 1996). Nevertheless, Bressa et al. (1999) considered that the neo-XY system of Dysdercus albofasciatus Berg, 1878; could support Ueshima's hypothesis.

Another cytogenetic characteristic of many heteropteran species is the presence of a minute chromosome pair with a special meiotic behaviour. These m chromosomes are generally observed unpaired during early meiotic prophase and no chiasma is detected between them. It is uncertain which the origin of this chromosome pair is and which is their function in the genetic system of the species possessing them (Ueshima 1979). On the basis of its presence in the primitive infraorder Dipsocoromorpha, Grozeva and Nokkala (1996) suggested that the m chromosomes were present in the ancestral karyotype of the Heteroptera.

During male meiosis sex chromosomes are identified from early prophase onwards because they are positively heteropycnotic. After pachytene many species present a diffuse stage during which the bivalents decondense and the nucleus increases its size and acquires an interphase appearance. Immediately after the diffuse stage the cell entres diakinesis and bivalents show one or sometimes two chiasmata. At metaphase I autosomal bivalents arrange in a circle with the sex univalents at its centre. The asynaptic sex chromosomes lie side by side, segregating sister chromatids at anaphase I. The first meiotic division is therefore reductional for autosomes and m chromosomes, and equational for the sex chromosomes. The second meiotic division follows immediately after the first one without interkinesis. At metaphase II the autosomes dispose at the periphery of the spindle while the sex chromosomes associate non-chiasmatically forming a pseudo-bivalent that localizes at the centre of the ring of autosomes. At anaphase II, the X and Y segregate to opposite poles and second division is reductional for the sex chromosomes (Ueshima 1979).

Cytogenetic reports show that most Pentatomidae present a diploid number 2n = 14 and a sex chromosome determining system XY/XX, without a pair of m chromosomes, and male meiosis follows this general pattern. However, one particular feature of some species of this family is the regular presence of an abnormal meiosis in one testicular lobe (harlequin lobe). Bowen (1922a,b cited in Ueshima 1979) and Schrader (1945a,b, 1946a,b, 1960a,b) were the first to report the presence of this harlequin lobe, and later Rebagliati et al. (2001) and Lanzone et al. (2003) also described it, summing up a total number of 22 species belonging to Pentatominae, Edessinae and Discocephalinae.

Materials and Methods

The species of Pentatomidae we have cytogenetically analysed are listed in Table 1. Adult males were analysed following standard procedures (Rebagliati et al. 2001).

Results and Discussion

The chromosome number and sex chromosome determining system in all the species and subspecies of Pentatomidae cytogenetically analysed until present are listed in Table 2. The number of species, diploid numbers and sex chromosome determining systems of superfamily Pentatomoidea are summarized in Table 3. The detail at subfamily and tribe level is given only for Pentatomidae.

The 28 species cytogenetically analysed from Argentine material present a meiotic behaviour that agrees with the previous description for the family. With reference to the diploid number 21 species present 2n = 14; four species [Dichelops furcatus (Fabricius, 1775), Dichelops melacanthus (Dallas, 1851), Mecocephala maldonadensis Schwertner, Grazia and Fernandes, 2002 and Acledra bonariensis (Stål, 1859)] present a reduced number (2n = 12) and another three species [Podisus distinctus (Stål, 1860), Podisus nigrispinus (Dallas, 1851) and Thyanta sp.1] possess an increased diploid number (2n = 16). All of them have a sex chromosome determining system XY/XX (Fig. 1). Among the analysed Argentinean species only Dinocoris prolineatus Becker and Grazia, 1985, Loxa deducta Walker, 1867 and Macropygium reticulare (Fabricius, 1803) present a harlequin lobe. The analysis of all of them suggests a common meiotic pattern. Among the meiotic traits that are altered in the harlequin lobe we can mention: meiotic pairing and chiasma formation, non-specific association of autosomes, an anomalous disposition of the chromosomes in the metaphase plate, chromosome segregation and cellular fusion. The final result is the production of sperm with an abnormal and highly variable chromosome number, which will affect the individual fertility (Fig. 2).

Within Pentatomidae 294 species and subspecies belonging to 121 genera from the subfamilies Asopinae, Discocephalinae, Edessinae, Pentatominae, Phyllocephalinae and Podopinae have been cytogenetically analysed. The male diploid numbers are between six and 27 with a mode in 14 chromosomes; the latter diploid number is present in 85% of the species. The sex chromosome determining system is XY/XX except in three species: M. reticulare, Rhytidolomia senilis (Say, 1832) and Thyanta calceata (Say, 1832) (Ueshima 1979; Manna and Deb-Mallick 1981; Nuamah 1982; Dey and Wangdi 1988; Satapathy and Patnaik 1988, 1989; Satapathy et al. 1990; Rebagliati 2000; Rebagliati et al. 2001, 2002, 2003; Lanzone et al. 2003; Kerzhner et al. 2004).

Within the subfamily Asopinae 22 species have been studied and the modal number is 2n = 14 (n = 6 + XY) with a reduction in chromosome number in Oechalia patruelis (Stål, 1859) and a population of Picromerus bidens (Linnaeus, 1758) (2n = 12, n = 5 + XY); and an increase in chromosome number in all the species of Podisus Herrich-Schaeffer, 1853 (2n = 16, n = 7 + XY) and in Dynorhynchus dybowskii Jakovlev, 1876 (2n = 18, n = 8 + XY). Within Discocephalinae the 18 species cytogenetically analysed present 2n = 14 (n = 6 + XY). In M. reticulare, Srivastava (1957) described 2n = 15 (n = 6 + X₁X₂Y, male)/2n = 16 (n = 6 +X₁X₁X₂X₂, females), although according to Rebagliati et al. (2001) the third sex chromosome should be a B chromosome. The 15 species of the subfamily Edessinae and the unique species of Phyllocephalinae cytogenetically analysed show a constant diploid number and sex chromosome determining system 2n = 14 (n = 6 + XY). Within the subfamily Pentatominae the study of 228 species reveals different diploid numbers from six in R. senilis (2n = 4 + neoX–neoY) to 27 in T. calceata (2n = 24 + X₁X₂Y) with a modal number of 14, and a sex chromosome determining system XY/XX (Wilson 1911; Schrader 1940; Schrader and Hughes-Schrader 1956). The other numbers described are 2n = 10 (in Oebalus pugnax Fabricius, 1775), 2n = 12 (n = 5 + XY) in A. bonariensis, D. furcatus, D. melacanthus, M. maldonadensis, Euschistus crassus Dallas, 1851 and Stagonomus bipunctatus (Linnaeus, 1758); 2n = 16 (n = 7 + XY) in 20 species of the genera Banasa Stål, 1860, Caystrus Stål, 1861, Cosmopepla Stål, 1867, Dunnius Distant, 1902, Eysarcoris Hahn, 1834, Gynenica Dallas, 1851, Niphe Stål, 1867, Palomena Mulsant and Rey, 1866, Spermatodes Bergroth, 1914 and Thyanta Stål, 1862, and 2n = 26 (n = 12 + XY) in 13 species of Banasa.

Among the 10 species of Podopinae only Scotinophara coarctata Fabricius, 1798 and Scotinophara sp. show a reduction in the diploid number (2n = 12, n = 5 + XY).

Phylogenetic relationships among the 16 families of Pentatomoidea are under investigation (J. Grazia, personal communication). From a cytogenetic point of view, Schaefer (1993) stated that the presence of a pair of m chromosomes in Thaumastellidae (Jacobs 1989), which is lacking in all other families of Pentatomoidea, would suggest a relationship with species of Lygaeoidea, in which m chromosomes are widely present. Henry (1997) suggested that the presence of m chromosomes, although lost in a few taxa (Berytidae, Lygaeinae, and Piesmatidae), is a synapomorphy defining a broad group within the Pentatomomorpha, and that the Thaumastellidae, because of the presence of a pair of m chromosomes, may not belong to the Pentatomoidea. On the other hand, many synapomorphies to the Pentatomoidea are found in the Thaumastellidae (e.g. presence of foretibial apparatus, developed mandibular plates, base of head not forming a ‘neck’, post-ocular tubercles absent, structure of the female genitalia, and paired lateral trichobothria) (J. Grazia, personal communication). Taking into account the hypothesis of Grozeva and Nokkala (1996), that m chromosomes might be included in the ancestral karyotype of Heteroptera, the absence of m chromosomes in the family Pentatomidae could be assumed as a derived character.

According to Manna (1984) from a plesiomorphic karyotype 2n = 14 (12 + XY) two branches evolved within Pentatomoidea; one of them includes the families that maintain this chromosome complement, e.g. Pentatomidae, while the other branch includes families with a reduction in diploid number. Acanthosomatidae, Cydnidae, Plataspididae, Scutelleridae and Tessaratomidae belong to this branch; all of them have a modal number 2n = 12 (10 + XY) which could suggest a common origin. Dinidoridae and Urostylididae present the plesiomorphic chromosome number, although some species present an increase in the chromosome number. In Thaumastellidae, the two species analysed present an increased diploid number (Table 3).

The multiple sex chromosome system X₁X₂Y/X₁X₁X₂X₂ has been reported in unrelated species within Pentatomoidea, suggesting that the origin of these derived systems should have been independent. Only one species has been reported with an XO system (Acanthosoma expansum Horvath, 1905) and it would be desirable to reanalyse it. Within Pentatomidae the increase in diploid number through fragmentations of autosomes and/or sex chromosomes has been more frequent than its reduction through chromosome fusions (11% versus 4% of the total number of species cytogenetically analysed). The great uniformity in chromosome number and sex determining system found in Pentatomidae indicates that speciation within this family is not accompanied by a process of karyotype change.

Until present a total number of 22 species of 15 genera of Pentatomidae have been reported that present an harlequin lobe with abnormal meiosis. Although there is a common pattern in the meiotic behaviour, all the species that bear it present peculiar traits (Bowen 1922a,b; Schrader 1945a,b, 1946a,b; Martin 1953; Srivastava 1957; Ansley 1958; Schrader 1960a,b; Rebagliati et al. 2001; Lanzone et al. 2003).

Rolston (1981) includes the genera Alitocoris Sailer, 1850, Macropygium Spinola, 1837, Melanodermus Stål, 1867 (Ochlerus Spinola, 1837), Moncus Stål, 1867 and Schraderia Ruckes, 1959 (Schraderiellus Rider, 1998) in the tribe Ochlerini (Discocephalinae), which were previously included in the tribe Halyini (Pentatominae). Although no direct relationship between the presence of the harlequin lobe and a particular taxonomic rank seems to exist, according to this new classification 11 of the 15 genera with harlequin lobe belong to Discocephalinae {only Discocephalessa humilis Herrich-Schaeffer, 1843 [as Platycarenus notulatus (Stål, 1862)] of this family does not possess it}. On the other hand, only Loxa Amyot and Serville, 1843 (three species) Mayrinia Horvath, 1925 (one species) and Adevoplitus Grazia and Becker, 1997 (Pseudevoplitus Ruckes, 1958) (one species) within Pentatomini (the tribe most vastly studied of Pentatominae) and Brachystethus rubromaculatus Dallas, 1851 within Edessinae show this abnormal meiosis. It is necessary to widen the cytogenetic analysis to more species from different regions in order to determine whether any relationship with the environment does exist, and to clarify its evolutionary and/or biological meaning.