Chiasmate male meiosis in six species of water bugs from infraorders Nepomorpha and Gerromorpha (Insecta: Heteroptera)

The type of male meiosis is a stable character at the family level in the order Heteroptera and provides additional information on the relationships between taxa. The most common pattern, probably ancestral in the order is chiasmate meiosis; however achiasmate meiosis has been described in fi ve families of terrestrial Heteroptera, all belonging to the infraorder Cimicomorpha (Anthocoridae, Microphysidae, Cimicidae, Miridae, Nabidae). Among water bugs, achiasmate meiosis is reported in the families Saldidae (Leptopodomorpha) and Мicronectidae (Nepomorpha). Regarding the third infraorder of water bugs, Gerromorpha, data on meiotic patterns are absent, except for the Limnogonus aduncus Drake, Harris, 1933 (Gerridae) possessing chiasmate meiosis. In this paper, the male meiotic pattern of six water bugs species from infraorders Nepomorpha (Plea minutissima minnutissima Leach, 1817) and Gerromorpha (Mesovelia furcata Mulsant, Ray, 1852, Microvelia reticulata (Burmeister, 1835), Gerris costae fi eberi Stichel, 1938, Hydrometra gracilenta Horváth, 1899, Velia pelagonensis Hoberlandt, 1941) is studied, and the karyotypes of the last two species are described for the fi rst time. In the species examined, bivalents are chiasmate, so all these species possess chiasmate meiosis in males.


MATERIAL AND METHODS
The data on the insects used in the present study are presented in the Table .The insects were fi xed alive in 3:1 fi xative (96% ethanol -glacial acetic acid mixture).The abdomen was dissected in 45% acetic acid and the internal reproductive system was analyzed under a stereomicroscope.The gonads were squashed in a small drop of 45% acetic acid.The cover slips were removed by a dry ice technique.Slides were dehydrated in fresh fi xative (3:1) and air dried.To study the number and the behavior of the chromosomes the preparations were stained by Schiff-Giemsa method after Grozeva, Nokkala (1996).Chromosome spreads were analyzed using a Laborlux 12 (Leitz, Wetzlar, Germany) and Olympus BХ 51 microscopes and documented by digital photo camera Moticam 2000.The preparations are preserved in the collection of the Laboratory "Cytotaxonomy and evolution", Institute of Zoology BAS, Sofi a.The karyotype of this species is described for the fi rst time.In Fig. 6, a spermatogonial metaphase displays 18 autosomes and the very large X chromosome, which is the largest chromosome of the set.The autosomes gradually decrease in size.In МІ, there are 9 bivalents with terminal chiasmata and a univalent X chromosome (Fig. 7).The chromatids of the X lie in parallel to the equator and are oriented to different poles Veliidae Microvelia reticulata (Burmeister, 1835), 2n=20+X.
Cobben (1968) interpreted the karyotype of M. reticulata as 21 (XO).In the present study we confi rmed this chromosome formula, as both MI and MII (Figs 8 and 9) show 11 chromosome elements.It is not possible to identify the X chromosome reliably.At МІ, all bivalents displayed terminal chiasmata (Fig. 8).
The karyotype of this species is studied for the fi rst time.Fig. 10 shows prometaphase І with 12 bivalents, every bivalent possessing a terminal or subterminal chiasma.At MI (Fig. 11), the autosomal bivalents gradually decrease in size, and the Х chromosome is one of the smallest in the set.The fi rst division, reductional for the autosomes but equational for the X chromosome, results in two kinds of TII cells: one with 12 autosomes and the daughter X, and the other with 12 autosomes only (Fig. 12).Stichel, 1938, 2n=20+X.Our observations confi rm the chromosome formula 2n=20+X earlier reported for this species by Sоuthwood and Leston (1959).At diakinesis (Fig. 13), autosomal bivalents
In the present study, the chromosome formula reported earlier for this species by Jande (1959Jande ( , 1961) ) was confi rmed.At prometaphase plates (PMI), most bivalents display one terminal chiasma, while the largest bivalent more often has two terminal chiasmata resulting in a ring (Fig. 1, a), or one subterminal chiasma (Fig. 1, b).Аt metaphase I (MI), 10 autosomal bivalents gradually decreasing in size, a pair of comparatively large m-chromosomes and the univalent X chromosome as large as the largest autosome can be seen (Fig. 2).The fi rst division is reductional for the autosomes, whereas the sex chromosome undergoes equational separation (post-reduction).As a result, at metaphase II (MII) daughter cells each contain 12 elements (10 autosomes, m-chromosome and the daughter X chromosome) (Fig. 3).
In this species, the testes were examined.Every testis consists of one elliptic orange follicle.Our study confi rmed the chromosome formula of this species published by Ekbolm (1941).At diakinensis, 15 bivalents each with one terminal or subterminal chiasma and fi ve univalent sex chromosomes can be seen (Fig. 4).At МІ, four Х and one Y sex chromosomes are placed inside the circle formed by the autosomal bivalents (Fig. 5).

Comp. Cytogenet., 2009 3(2)
Comparative Cytogenetics with terminal or interstitial chiasmata and the X chromosome can be seen.At МІ (Fig. 14) and МІІ (Fig. 15), the X chromosome is easily recognized because it is heteropycnotic and one of the largest chromosomes in the set.Fig. 14 shows a MІ plate, in which the chromatids of the X lie in parallel to the equator and are oriented to different poles.
Although there are no reviews dedicated to the type of meiosis in the water bugs (Nepomorpha, Gerromorpha, Leptopodomorpha), it can be supposed that the species with chiasmate meiosis also predominate in these groups.In two Saldula Van Duzee, 1914 species (Saldidae) achiasmate meiosis has been earlier reported (Nokkala, Nokkala, 1983), but there are no data on the meiosis of the three other families of the infraorder Leptopodomorpha.Recently, achiasmate meiosis has been reported for the fi rst time in the infraorder Nepomorpha.In this infraorder, species of the family Corixidae display chiasmata during the male meiosis (Waller, Angus, 2005), but four species of the family Мicronectidae (Tenagobia (Fuscagobia) fuscata (Stål, 1859), Micronecta scholtzi (Fieber, 1860), M. poweri (Douglas et Scott, 1869) and M. griseola Horvath, 1899) were found to have achiasmate male meiosis (Ituarte, Papeschi, 2004;Grozeva et al., 2008), These data elucidate the taxonomic problems within the Corixoidea, and provide additional support for the idea by Nieser (2002) to consider Micronectidae separate from the family Corixidae.
Recently, in the infraorder Gerromorpha terminal and interstitial chiasmata were reported for two Hebrus Curtis, 1833 species (Hebridae) (Nokkala, Nokkala, 1999), as well as for Limnogonus aduncus Drake, Harris, 1933 (Gerridae) (Castanhole et al., 2008).However, for the other over twenty species examined only the number of chromosomes is given (see Ueshima, 1979) without any information about the type of meiosis in males.
In the present study, chiasmata were described in six water bugs species, belonging to one family of the Nepomorpha (Pleidae) and four families of the Gerromorpha (Hydrometridae, Veliidae, Mesoveliidae, Gerridae).Thеse observations confi rm the suggestion that chiasmate male meiosis is the most common and probably ancestral meiotic pattern in the order Heteroptera, well distributed among both the terrestrial and the water bugs.Additional studies of different heteropteran taxa, and especially of water bugs, are necessary to learn more about the distribution of the meiotic patterns among the true bugs as a whole.

Table .
Material used for chromosome analysis.