Journal of Biological Research 5: 3 – 21, 2006 J. Biol. Res. is available online at http://www.jbr.gr — INVITED REVIEW — Evolutionary cytogenetics in Heteroptera ALBA GRACIELA PAPESCHI* and MARI´A JOSE´ BRESSA Laboratorio de Citogenética y Evolucio´ n, Departamento de EcologÈ´a, Genética y Evolucio´ n, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina Received: 2 September 2005 Accepted after revision: 29 December 2005 In this review, the principal mechanisms of karyotype evolution in bugs (Heteroptera) are dis- cussed. Bugs possess holokinetic chromosomes, i.e. chromosomes without primary constriction, the centromere; a pre-reductional type of meiosis for autosomes and m-chromosomes, i.e. they segregate reductionally at meiosis I; and an equational division of sex chromosomes at anaphase I. Diploid numbers range from 2n=4 to 2n=80, but about 70% of the species have 12 to 34 chromosomes. The chromosome mechanism of sex determination is of the XY/XX type (males/females), but derived variants such as an X0/XX system or multiple sex chromosome sys- tems are common. On the other hand, neo-sex chromosomes are rare. Our results in het- eropteran species belonging to different families let us exemplify some of the principal chromo- some changes that usually take place during the evolution of species: autosomal fusions, fusions between autosomes and sex chromosomes, fragmentation of sex chromosomes, and variation in heterochromatin content. Other chromosome rearrangements, such as translocations or inver- sions are almost absent. Molecular cytogenetic techniques, recently employed in bugs, represent promising tools to further clarify the mechanisms of karyotype evolution in Heteroptera. Key words: holokinetic chromosomes, karyotype evolution, molecular cytogenetics. INTRODUCTION between 1897 and 1932, Wilson and Montgomery settled the foundations on Heteroptera cytogenetics. Cytogenetic studies in Heteroptera date from 1891 Some years later, between the 1930s and the 1960s, with Henking’s morphological study on the sper- Schrader and Hughes-Schrader contributed signifi- matogenesis of the bug Pyrrhocoris apterus Linnaeus cantly to a better knowledge on the organization and (Pyrrhocoridae). He described the presence of a function of heteropteran chromosomes and their chromatin body (the “X chromosome”) that showed behaviour during cell divisions (Ueshima, 1979; Man- an unusual staining during meiotic prophase and a na, 1984). peculiar behaviour during meiosis. The observations At present, although approximately 1600 het- of Henking and other cytologists, together with their eropteran species belonging to 46 families have been own observations in males and females of different cytogenetically analyzed, they are not a representa- Heteroptera species, were finally interpreted by Mc tive sample of the group (4.2% of approximately Clung (1901) and Wilson (1909b). They suggested a 38,000 described species) (Table 1) (Schuh & Slater, direct relationship between sex determination and 1995). Most studies have been made on male testes, presence of either one or two “X chromosomes” in because meiotic cells are more often encountered X0/XX systems, or an XY or XX chromosome pair than mitotic ones, and the earliest reports referred to in species with XY/XX systems. the male chromosome number and sex chromosome Through a series of publications that appeared system. No detailed analysis of heteropteran chro- mosome morphology is possible, since chromosomes * Corresponding author: tel.: +54 11 45763354, fax: +54 are holokinetic (a primary constriction, the centro- 11 45763384, e-mail: [email protected] mere, is lacking) and the chromosomes are generally 3 4 A. G. Papeschi and M. J. Bressa — Evolutionary cytogenetics in Heteroptera relatively small and of similar size. Although DNA regarded as telokinetic (Motzko & Ruthmann, 1984). measurement is the best means of testing fusion/frag- Both chromosome ends can show kinetic activity mentation hypothesis in karyotypic evolution of taxa during meiosis in such a way that the telomeric bearing holokinetic chromosomes, this technique has regions that were inactive during the first meiotic been rarely used in Heteroptera (Schrader & Hughes- division become active during the second one (Cama- Schrader, 1956, 1958; Maudlin, 1974; Papeschi, 1988, cho et al., 1985; Nokkala, 1985; Pérez et al., 1997; Cat- 1991; Panzera et al., 1992, 1995; Papeschi et al., 2001; tani et al., 2004). The holokinetic nature of het- Grozeva & Nokkala, 2003). Furthermore, cytogenet- eropteran chromosomes has led to the hypothesis ic studies at the population level in Heteroptera are that fusions and fragmentations would be the most still scarce. All these facts represent serious con- frequent mechanisms of karyotype evolution. The straints in comparative and evolutionary cytogenetics non-localized kinetic activity does not have the con- of this insect group. straints imposed by a localized centromere in species Recently, molecular approaches such as fluores- with monocentric chromosomes. In the latter, acen- cent banding and fluorescent in situ hybridization tric fragments or dicentrics are mitotically and meiot- with telomeric and ribosomal DNA probes (18S and ically unstable. 28S rDNA), have been carried out in bugs (Gonza´lez- The meiotic behaviour of autosomal bivalents, sex García et al., 1996; Sahara et al., 1999; Papeschi et al., chromosomes and m chromosomes is slightly differ- 2003; Rebagliati et al., 2003; Cattani & Papeschi, ent. As a rule, autosomal bivalents are chiasmatic, 2004; Cattani et al., 2004; Frydrychova´ et al., 2004; whereas sex chromosomes and m chromosomes are Grozeva et al., 2004; Hebsgaard et al., 2004; Ituarte & achiasmatic (Ueshima, 1979; Manna, 1984; Gonza´lez- Papeschi, 2004; Bressa et al., 2005; Vítkova´ et al., García et al., 1996; Suja et al., 2000). When autosomal 2005). These molecular techniques have substantial- bivalents form only one chiasma terminally located, ly contributed to a better understanding of the chro- they orientate at metaphase I with their long axes matin organization and composition of these holoki- parallel to the polar axis; they segregate reductional- netic chromosomes. However, there is still much to ly during meiosis I and equationally during meiosis II. be learnt before the mechanisms of karyotype evolu- On the other hand, bivalents with two terminal chias- tion and the role in the speciation of heteropterans is mata orientate equatorially and two different behav- completely clarified. iours have been described: a) one chiasma releases first and an axial orientation is finally achieved or b) CYTOGENETICS OF HETEROPTERA: alternative sites of kinetic activity become functional GENERAL FEATURES and no telokinetic activity is observed (Mola & Pa- peschi, 1993; Papeschi et al., 2003). This latter behav- The genetic system of Heteroptera has many charac- iour has been described in Pachylis argentinus Berg teristics that make it unique among most insect and Nezara viridula (Linnaeus); both species possess groups: the possession of holokinetic chromosomes, a pair of chromosomes carrying a secondary constric- a different meiotic behaviour for autosomes and sex tion, and when this particular pair presented two chi- chromosomes, a mean chiasma frequency of only one asmata, the alternative kinetic sites were located next chiasma per bivalent, and a pair of “m chromosomes” to the secondary constrictions (Papeschi et al., 2003). in some families. When chiasmata are medially located, the terms pre- All heteropteran species possess holokinetic chro- reduction and post-reduction lose their meaning. The mosomes (i.e. without a localized centromere). genetic information located at the chiasma to one During mitosis, microtubules attach to the entire pair of telomeric regions divides reductionally, while length of the sister chromatids and at anaphase, they the information located at the other half of the chro- migrate parallel to each other and perpendicular to mosome does so equationally. We consider meiosis in the polar spindle (Hughes-Schrader & Schrader, Heteroptera as pre-reductional taking into account 1961). At the ultrastructural level, kinetochore plates, the behaviour of bivalents with one terminal or sub- to which spindle microtubules are attached, cover terminal chiasma, in which most genetic information almost all the polar surface of the mitotic chromo- segregates reductionally at meiosis I; and the behav- somes (Buck, 1968; Comings & Okada, 1972). How- iour of fusion/fragmentation trivalents, in which the ever, kinetic activity during meiosis is restricted to the largest chromosome always segregates from the two telomeric regions and the chromosomes can be smaller ones at meiosis I (Ueshima, 1963; Papeschi, A. G. Papeschi and M. J. Bressa — Evolutionary cytogenetics in Heteroptera 5 1994). A post-reductional meiosis has been described rochromatic. The different pycnotic cycle of the m in other groups with holokinetic chromosomes (e.g. chromosomes reflects differences in chromatin pack- Juncaceae, Odonata) (Nordenskiöld, 1962; Mola, aging, and this pattern of chromatin condensation is 1995). probably related to the regulation of gene expression On the other hand, sex chromosomes are achias- (Bressa et al., 2005). However, there is still no infor- matic and behave as univalents in male meiosis I. mation on the genetic content of the m chromo- They divide equationally at anaphase I and associate somes. at meiosis II through the