Chromosome numbers in antlions (Myrmeleontidae) and owlflies (Ascalaphidae) (Insecta, Neuroptera).

A short review of main cytogenetic features of insects belonging to the sister neuropteran families Myrmeleontidae (antlions) and Ascalaphidae (owlflies) is presented, with a particular focus on their chromosome numbers and sex chromosome systems. Diploid male chromosome numbers are listed for 37 species, 21 genera from 9 subfamilies of the antlions as well as for seven species and five genera of the owlfly subfamily Ascalaphinae. The list includes data on five species whose karyotypes were studied in the present work. It is shown here that antlions and owlflies share a simple sex chromosome system XY/XX; a similar range of chromosome numbers, 2n = 14-26 and 2n = 18-22 respectively; and a peculiar distant pairing of sex chromosomes in male meiosis. Usually the karyotype is particularly stable within a genus but there are some exceptions in both families (in the genera Palpares and Libelloides respectively). The Myrmeleontidae and Ascalaphidae differ in their modal chromosome numbers. Most antlions exhibit 2n = 14 and 16, and Palparinae are the only subfamily characterized by higher numbers, 2n = 22, 24, and 26. The higher numbers, 2n = 20 and 22, are also found in owlflies. Since the Palparinae represent a basal phylogenetic lineage of the Myrmeleontidae, it is hypothesized that higher chromosome numbers are ancestral for antlions and were inherited from the common ancestor of Myrmeleontidae + Ascalaphidae. They were preserved in the Palparinae (Myrmeleontidae), but changed via chromosomal fusions toward lower numbers in other subfamilies.

Introduction

Within the holometabolous (= Endopterygota) insect order (lacewings) including a total of 17 or 18 currently recognized families (Aspöck et al. 2012), the (antlions) comprise the most species-rich and most widespread family, with over 1500 valid extant species in 191 genera (Stange 2004). The closely related (owlflies) are a moderately speciose neuropteran family encompassing approximately 400 valid extant species assigned to about 65 genera, with wide distributional range in tropical and temperate areas of the world (Sekimoto and Yoshizawa 2007).

The and belong to the superfamily (suborder ), together with another four extant families, , , , and . Despite the controversial hypotheses on the interfamilial phylogenetic relationships within this group, different phylogenetic analyses based on morphological and genetic data provide almost universal support for the monophyly of and the sister relationship between and (Stange 1994, Aspöck 2002, Haring and Aspöck 2004, Winterton et al. 2010, Aspöck et al. 2012). However, molecular analyses are not always concordant with the monophyly of these families (Winterton et al. 2010).

Within , the higher-level classification is controversial (reviewed in Mansell 1999), with several authors proposing various taxonomic divisions at the subfamily, tribe and subtribe levels (e.g. Banks 1899, 1927, New 1985a, b, c, Stange 1994, 2004, Krivokhatsky 2011). In his recent monography on the world fauna of , Stange (2004) recognized three subfamilies, , , and , with 14 tribes and 191 genera. were further classified by Krivokhatsky (2011) who subdivided the family into 12 subfamilies (, , , , , , , , , , , and ), with 23 tribes.

The are poorly-understood and taxonomically weakly-elaborated family. It was extensively revised only by van der Weele (1908) and now it comprises at least three subfamilies, (now ), (now ), and , with a total of 15 tribes. Two-thirds of the species are placed in the first subfamily, and the remaining species (approximately 90) are placed in the second one, whereas the third subfamily contains only one species (van der Weele 1908, Sekimoto and Yoshizawa 2007). To date, no wide-ranging modern phylogenetic analyses of higher ascalaphid relationships have been published (Fischer et al. 2006).

Mansell (1999: p. 3) pointed out that the antlions, “apart from their obvious biological significance, are ideal subjects for the study of insect behavior, physiology, biogeography and evolution, and consequently a group urgently warrants study and conPageBreakservation”. Although chromosomal investigations have a long history in systematics and evolutionary biology (White 1973, King 1993), and a large body of data has been accumulated for insects (e.g., butterflies: Lukhtanov 2014; beetles: Angus et al. 2013, Blackmon and Demuth 2014, 2015; true bugs: Papeschi and Bressa 2006, Kuznetsova et al. 2011; aphids: Gavrilov-Zimin et al. 2015; coccids: Gavrilov 2007; cicadas: Kuznetsova and Aguin-Pombo 2015; grasshoppers: Warchałowska-Śliwa et al. 2005, parasitic wasps: Gokhman 2009), both antlions and owlflies were largely ignored in this respect. Our present knowledge of their karyotypes is scarce and fragmentary, being completely confined to the number of chromosomes and, additionally, to the meiotic behavior of the sex chromosomes that is of a very peculiar type in many neuropteran groups (Naville and de Beaumont 1932, 1933, Hughes-Schrader 1969, 1975a, b, 1979, Nokkala 1986) including the (Naville and de Beaumont 1932, 1933, Hughes-Schrader 1983). In the and , chromosomal studies were initiated in the 1930s with the pioneering works of Oguma and Asana (1932), Naville and de Beaumont (1932, 1933, 1936), Ikeda and Kichijo (1935), Asana and Kichijo (1936), and Katayama (1939). Since that time only scarce chromosome studies were performed on the (Hirai 1955a, b, Highes-Schrader 1983, Klok and Chown 1993) while no further work on the appeared except for the re-investigation of (Walker, 1853) karyotype (Hirai 1955a, b: as Lefèbvre, 1842) earlier studied by Katayama (1939: as ).

Thus, cytogenetic studies on the families and virtually ceased a few decades ago. The latest checklist of chromosome numbers in antlions published by Klok and Chown (1993) suffers from many shortcomings including imperfect references, erroneous identifications, outdated species names and synonymy. In order to fill this gap, an updated and comprehensive checklist of chromosome numbers of antlions and owlflies is provided here by integrating the published data together with our latest unpublished results.

Material and methods

Insects

Four antlion species (only males), namely , , , , and male owlfly , were used in the present study. The specimens were collected from May to October 2013 in the Republic of Dagestan (North-East Caucasus, Russia). The material was collected by G. Khabiev. Collection sites, sampling dates, and the numbers of studied males are given in Table 1. In the field, adult individuals were fixed in a solution of 96% alcohol and glacial acetic acid (3:1) and then stored at 4 °C until required.

Table 1.

Material used.

Taxon	Sampling locality and date of collection	No. of studied males
Myrmeleontidae
Palparinae
Palpares libelluloides (Linnaeus, 1764)	Russia, Dagestan, near Makhachkala 43°00'00"N, 47°13'33"E; V.2013	2
Nemoleontinae
Distoleon tetragrammicus (Fabricius, 1798)	Russia, Dagestan, near Makhachkala 43°00'29"N, 47°14'51"E VII.2013	1
Macronemurus bilineatus Brauer, 1868	Russia, Dagestan, near Makhachkala 42°59'58"N 47°13'30"E; VI.2013	7
Myrmecaelurinae
Myrmecaelurus trigrammus (Pallas, 1771)	Russia, Dagestan, near Makhachkala 43°01'26"N, 47°15'12"E; 42°57'19"N, 47°28'51"E; 42°58'07.2"N, 47°20'03"E; VI-VII.2013	23
Ascalaphidae
Bubopsis hamatus (Klug in Ehrenberg, 1834)	Dagestan, Gumbetovsky district, near Chirkata village; 42°47'53"N, 46°41'14"E; VII.2013	2

Chromosome preparation

Air-dried preparations were made by macerating testicular follicles in a drop of 45% acetic acid on a glass microscope slide and squashing under a cover slip. The preparations were frozen using dry ice, the cover slips were removed with a razor blade, and the preparations were dehydrated in fresh fixative (3:1) for 20 min and air dried. Slides were first examined under a phase-contrast microscope to check for the availability of meiotic divisions and quality of chromosome spreads. Counts were based on samples of one to 23 individuals. The preparations and remains of the specimens are stored at the Department of Karyosystematics, Zoological Institute, RAS.

Chromosome staining

Meiotic chromosomes were stained using the Feulgen-Giemsa method developed by Grozeva and Nokkala (1996).

Microscopy and imaging

Chromosome preparations were analyzed under a Leica DM 4000B microscope with a 100x objective. Images were taken with a Leica DFC 345 FX camera using Leica Application Suite 3.7 software with an Image Overlay module.

Results

Only meiotic divisions in adult males were available for analysis during the present study. In five examined species belonging to the families (four species) and (one species) (Table 1), as many as three different chromosome numbers were found. Males of showed 12 autosomal bivalents and univalent X and Y chromosomes at suggesting the diploid karyotype formula of this species is 2n = 26(24A + XY). Unfortunately, our method proved to be inappropriate for effective and reliable detection of the centromere positions in chromosomes and, hence, analysis of their morphology. Nonetheless, most autosomes were suggested to be one-armed, with at least one clear exception of a large pair of bi-armed submetacentric chromosomes (Fig. 1). Males of and showed 7 autosomal bivalents and univalent X and Y chromosomes at spermatocyte MI suggesting the diploid karyotype formula is 2n = 16(14A + XY). Males of and showed 8 autosomal bivalents and univalent X and Y chromosomes at spermatocyte MI suggesting the diploid karyotype formula is 2n = 18(16A + XY). In the four low-numbered species, the chromosomes seemed to be essentially bi-armed (Figs 2–5).

Figures 1–5.

Meiotic (MI) karyotypes of antlions (1–4) and owlflies (5). 1 , n = 12AA+XY (2n = 26, XY) 2 , n = 8AA+XY (2n = 18, XY) 3 , n = 7AA+XY (2n = 16, XY) 4 , n = 7AA+XY (2n = 16, XY), 5 , n = 8AA+XY (2n = 18, XY). Arrows point to X and Y sex chromosomes. Scale bars = 10 µm

spermatocyte metaphases I

A peculiar feature of all the species was that at metaphase I, the univalent X and Y chromosomes were disposed on the opposite sides of the division spindle whereas autosomal bivalents showed a typical metaphase location on the equator of the nucleus (Figs 1–5). In each species, the behavior of sex chromosomes was traced in the meiotic nuclei throughout all stages and these data will be presented elsewhere.

The new findings and references to previous reports of chromosome numbers in and are given in Table 2. The subfamilial and tribal classification of the used in this paper follows Krivokhatsky (2011) and that of the follows van der Weele (1908).

Table 2.

Data on karyotypes in the and the (: ).

No	Taxon	2n (karyotype formula) ♂	Sampling locality	Reference
	Family Myrmeleontidae Latreille, 1802
	Subfamily Palparinae Banks, 1911
1	Indopalpares pardus (Rambur, 1842)	24(22+XY)	East India: Ahmedabad	Oguma and Asana 1932 (as Palpares sp.)1
2	Palpares libelluloides (Linnaeus, 1764)	26(24+XY) 26(24+XY)	Switzerland: Ge♀neve, France: Banyuls-sur-Mer Russia: Dagestan	Naville and De Beaumont 1936 Present data
3	Palpares sobrinus Péringuey, 1911	22(20+XY)	South Africa: Transvaal	Klok and Chown 1993
	Subfamily Pseudimarinae Markl, 1954
	Tribe Palparidiini Markl, 1954
4	Palparidius concinnus Péringuey, 1910	18(16+XY)	South Africa: Transvaal	Klok and Chown 1993
	Subfamily Dendroleontinae Banks, 1899
	Tribe Dendroleontini Banks, 1899
5	Epacanthaclisis moiwanus (Okamoto, 1906)	16(14+XX) (♀)	Japan	Hirai 1955a, b
6	Dendroleon jezoensis Okamoto, 1910	16(14+XY)	Japan	Hirai 1955a, b
	Subfamily Nemoleontinae Banks, 1911
	Tribe Distoleontini Tillyard, 1916
7	Distoleon tetragrammicus (Fabricius, 1798)	18(16+XY)	Russia: Dagestan	Present data
	Tribe Neuroleontini Banks, 1911
8	Neuroleon sp.2	16(14+XY)	Western India: Bombay [Mumbai]	Asana and Kichijo 1936
	Tribe Macronemurini Esben-Petersen, 1919
9	Macronemurus appendiculatus (Latreille, 1807)	16(14+XY)	France: Banyuls-sur-Mer	Naville and De Beaumont 1933
10	Macronemurus bilineatus Brauer, 1868	16(14+XY)	Russia: Dagestan	Present data
11	Macronemurus sp.	16(14+XY)	Western India: Bombay [Mumbai]	Asana and Kichijo 1936 (as Macronemurus sp.?)
	Tribe Creoleontini Markl, 1954
12	Creoleon lugdunensis (Villers, 1789)	18(16+XY)	France: Banyuls-sur-Mer	Naville and De Beaumont 1936 (as Creagris plumbea Navás, 1928)3
	Subfamily Glenurinae Banks, 1927
	Tribe Glenurini Banks, 1927
13	Euptilon arizonensis (Banks, 1935)	16(14+XY)	USA	Hughes-Schrader 1983 (as Psammoleon arizonensis Banks, 1935)
14	Paraglenurus japonicus (MacLachlan, 1867)	16(14+XY)	Japan	Hirai 1955a, b (as Glenuroides japonicus MacLachlan, 1867)
	Subfamily Myrmeleontinae Latreille, 1802
	Tribe Myrmeleontini Latreille, 1802
15	Baliga micans (McLachlan, 1875)	14(12+XY)	Japan	Hirai 1955a, b (as Hagenomyia micans McLachlan, 1875)
16	Baliga sagax (Walker, 1853)	14(12+XY)	Western India: Bombay [Mumbai]	Asana and Kichijo 1936 (as Myrmeleon sp., probably Myrmeleon sagax Walker, 1853)
17	Euroleon nostras (Fourcroy, 1785)	14(12+XY)14(12+XX) (♀)	Switzerland, Geneva	Naville and De Beaumont 1932, 1933 (as Myrmeleon europaeus McLachlan, 1873)
18	Myrmeleon alcestris Banks, 1911	14(12+XY)	South Africa: Transvaal	Klok and Chown 1993
19	Myrmeleon californicus Banks, 1943	14(12+XY)	USA	Hughes-Schrader 1983
20	Myrmeleon exitialis Walker, 1853	14(12+XY)	USA	Hughes-Schrader 1983
21	Myrmeleon formicarius Linnaeus, 17672	14(12+XY) 14(12+XY)	Western India: Bombay [Mumbai] Japan	Ikeda and Kichijo 1935, Hirai 1955a, b
22	Myrmeleon hyalinus Olivier, 1811	14(12+XY)	France: Corse	Naville and De Beaumont 1936 (as Morter hyalinus (Olivier, 1811))
23	Myrmeleon immaculatus DeGeer, 1773	14(12+XY)	USA	Hughes-Schrader 1983
24	Myrmeleon mexicanus Banks, 1903	14(12+XY)	USA	Hughes-Schrader 1983
25	Myrmeleon obscurus Rambur, 1842	14(12+XY)	South Africa: Transvaal	Klok and Chown 1993
	Subfamily Brachynemurinae Banks, 1927
	Tribe Brachynemurini Banks, 1927
26	Brachynemurus hubbardi Currie, 1898	14(12+XY)	USA	Hughes-Schrader 1983
27	Brachynemurus mexicanus Banks, 1895	14(12+XY)	USA	Hughes-Schrader 1983
28	Clathroneuria coquilletti (Currie, 1898)	14(12+XY)	USA	Hughes-Schrader 1983 (as Brachynemurus coquilletti Currie, 1898)
29	Clathroneuria schwarzi (Currie, 1903)	14(12+XY)	USA	Hughes-Schrader, 1983 (as Brachynemurus schwarzi Currie, 1903)
30	Scotoleon dissimilis (Banks, 1903)	16(14+XY)	USA	Hughes-Schrader 1983 (as Brachynemurus dissimilis Banks, 1903)
31	Scotoleon niger (Currie, 1898)	16(14+XY)	USA	Hughes-Schrader 1983 (as Brachynemurus niger Currie, 1898)
32	Scotoleon nigrilabris (Hagen, 1888)	16(14+XY)	USA	Hughes-Schrader 1983 (as Brachynemurus nigrilabris Hagen, 1888)
	Subfamily Myrmecaelurinae Esben-Petersen, 1919
	Tribe Myrmecaelurini Esben-Petersen, 1919
33	Myrmecaelurus sp.2	14(12+XY)	Western India: Bombay [Mumbai]	Asana and Kichijo 1936 (as Myrmecaelurus sp, probably Myrmecaelurus acerbus (Walker, 1853))
34	Myrmecaelurus trigrammus (Pallas, 1771)	16(14+XY)	Russia: Dagestan	Present data
	Subfamily Acanthaclisinae Navás, 1912
35	Synclisis japonica (McLachlan, 1875)	14(12+XY) 14(12+XY)	Western India: Bombay [Mumbai] Japan	Ikeda and Kichijo 1935, Hirai 1955a, b (as Acanthaclisis japonica Hagen, 1866)
36	Centroclisis brachygaster (Rambur, 1842)	14(12+XY)	South Africa: Transvaal	Klok and Chown 1993
37	Vella fallax (Rambur, 1842)	14(12+XY)	USA	Hughes-Schrader 1983
	Family Ascalaphidae Rambur, 1842
	Subfamily Ascalaphinae Rambur, 1842
	Tribe Hybrisini Lefèbvre, 1842
38	Ascalohybris subjacens (Walker, 1853)	22(20+XY) 22(20+XX) (♀)	JapanJapan	Katayama 1939 (as Hybris subjacens (Walker, 1853)), Hirai 1955a, b (as Hybris subjacens)
39	G1yptobasis dentifera (Westwood, 1847)	22(20+XY)	Western India: Bombay [Mumbai]	Asana and Kichijo 1936
	Tribe Ascalaphini Rambur, 1842
40	Libelloides corsicus Rambur, 1842)	20	France: Corse	Naville and De Beaumont 1936 (as Ascalaphus ictericus corsicus Rambur, 1842)
41	Libelloides coccajus (Denis & Schiffermüller, 1775)	22(20+XY)22(20+XX) (♀)	Switzerland: Geneva, Valais	Naville and De Beaumont 1933, 1936 (as Ascalaphus libelluloides Schäffer, 1763)
42	Libelloides longicornis (Linnaeus, 1764)	22(20+XY)	Switzerland: Valais	Naville and De Beaumont 1936 (as Ascalaphus longicornis (Linnaeus, 1764))
	Tribe Encyoposini McLachlan, 1871
43	Bubopsis hamatus (Klug in Ehrenberg, 1834)	18(16+XY)	Russia: Dagestan	Present data
44	Ogcogaster segmentator (Westwood, 1847)	22(20+XY)	Western India: Bombay [Mumbai]	Asana and Kichijo 1936

Later described as Kuwayama, 1933 (Oguma and Asana 1932, Kuwayama 1933)

Presence of these taxa in Bombay [Mumbai] is doubtful

Wrong identifications: all records of Creoleon plumbeus from West Europe actually belong to (Hölzel 1976, Krivokhatsky 2011)

Discussion

Chromosome numbers

In the , with the original data presented here, karyotype data have been made available for 37 species and 21 genera in 9 out of 12 subfamilies accepted by Krivokhatsky (2011). Having regard to 1500 valid species and 191 valid genera in this family (Stange 2004), the proportion of the studied species and genera is approximately 2.5% and 11% respectively. The karyotypes (chromosome numbers and sex chromosome systems) are presently known for the subfamilies (3 species/2 genera), (1/1), (2/2), (6/4), (2/2), (11/3), (7/3), (2/1), and (3/3). The family demonstrates a relatively high diversity of karyotypes, with diploid chromosome numbers (2n) of 37 studied species ranging PageBreakPageBreakPageBreakPageBreakPageBreakfrom 14 to 26 including four intermediate counts, i.e. 16, 18, 22 and 24. The highest numbers, 26, 24 and 22, occur only in the subfamily , in three species of the genera Rambur, 1842 and Insom & Carfi, 1988. Other numbers, 2n = 14, 16 and 18, are encountered in the remaining subfamilies. In the , the only studied species, , exhibits the next highest number found in antlions, i.e. 2n = 18. In the , with the exception of and displaying 2n = 18, all studied species, including three Costa, 1855 species, have karyotypes with 2n = 16. The latter value is also found in all studied and . The subfamilies , and include species both with 2n = 16 and 2n = 14, whereas show 2n = 14 in all the studied species. It is noteworthy that, with the exception of (but see below), all these genera do not show interspecific variation in the chromosome number. This is especially remarkable for those genera where several species have been studied, e.g. () and Banks, 1913 (). In each of these genera, three studied species share 2n = 16. Moreover, in Linnaeus, 1767 () all eight studied species have 2n = 14. It is noteworthy that closely related genera, Navás, 1912 and Esben-Petersen, 1918 in the , show the same karyotype with 2n = 14. It is unclear at present whether the chromosome number varies within the genus . The highest chromosome number, 2n = 26, is found in , the type species of the genus. Kuwayama, 1933 with 2n = 24 (Oguma and Asana 1932) is treated here as a member of . Additionally, there is a possibility that with 2n = 22 (Klok and Chown 1993) represents in fact (McLachlan, 1867). Although few members of the are studied at present, karyotypic differences between the genera of this subfamily probably occur.

Compared to the , karyotypes of the are less studied. The chromosome numbers are currently known in only seven owlfly species from the genera Sziraki, 1998, Westwood, 1847, Schaeffer, 1766, McLachlan, 1898, and McLachlan, 1871, all presently classified within the subfamily . The species studied show relatively high chromosome numbers, i.e. 2n = 18 in , 20 in , and 22 in all the remaining species, including two other studied members of the genus .

Although and show a similar range of chromosome numbers (2n = 14 - 26 in the former and 18 - 22 in the latter), these families differ in the modal numbers. Of 37 species studied in the , 19 species display 2n = 14, and 12 species have 2n = 16. On the other hand, five of seven species studied in the display 2n = 22. Other chromosome numbers occur only occasionally within the families except for high numbers characteristic of the antlion subfamily .

In different eukaryotic organisms, evolutionary changes in the chromosome number happen via polyploidy, aneuploidy or fusion/fission events. In animals polyploidy is known to be rare, whereas chromosomal fusions and fissions are common. As stated PageBreakabove, most possess lower chromosome numbers, 2n = 14 and 2n = 16, which are encountered in all subfamilies, with the only exception of the . The latter is the only subfamily characterized by higher numbers, 2n = 26, 24, and 22, and the higher number, 2n = 18, is also found in the only studied species of the related subfamily . The higher numbers, 2n = 22, 20 and 18, are also characteristic of the sister family . Since represent a basal phylogenetic lineage of the (Krivokhatsky 2011), it is hypothesized that higher chromosome numbers are ancestral for antlions. Most likely, the higher chromosome numbers were inherited from the common ancestor of + . It was preserved in the subfamily () but changed via chromosomal fusions toward lower numbers, 2n = 18, 16 and 14, in other subfamilies.

Knowledge of the chromosome morphology in the low-numbered and high-numbered chromosome complements would help in understanding the karyotype evolution in the and and testing the above hypothesis. Unfortunately, despite several efforts to identify chromosomal morphology within particular karyotypes (e.g. Asana and Kichijo 1936, Hughes-Schrader 1983, present study), this important question remains unresolved. Special staining methods, e.g. C-banding, are therefore needed to identify the centromeric position in the chromosomes and thus their morphology. However, these techniques have never been used in neuropteran cytogenetics, and therefore this is the most serious objective in the chromosome research of antlions and owlflies.

Sex chromosome system

All and species, including those studied here, exhibit a simple sex chromosome system XY/XX, which is characteristic of the whole order (White 1973, Blackman 1995). Both antlions and owlflies demonstrate a very peculiar behavior of sex chromosomes in males (Naville and de Beaumont 1933, 1936, Asana and Kichijo 1936, Hughes-Schrader 1983, Klok and Chown 1993, present paper). In spermatocyte meiosis of those insects, sex chromosomes take up positions at opposite halves of the meiotic spindle at metaphase I before segregating into the daughter spermatocytes. It means that the X and Y chromosomes get segregated to opposite poles of the spindle long before the autosomal half-bivalents disjoin at anaphase I and move to the poles. The same pattern, the so-called “distance pairing” of sex chromosomes first discovered by Naville and de Beaumont (1933) in antlions, is known to be characteristic of the related neuropteran families , , , , and (Naville and de Beaumont 1936, Hughes-Schrader 1969, 1975b, 1980, Nokkala 1986) and probably of the order in general. The biological role of this unusual behavior of sex chromosomes is unclear. In any case, this mechanism observed in brown lacewings () showed no significance for the regular segregation of the sex chromosomes in meiosis (Nokkala 1986).

The order belongs to the superorder , which comprises another two orders, namely, with two extant families, and , and with two extant families, and (Aspöck and Aspöck 2007). Interestingly, the share the “distance pairing” of sex chromosomes with (Naville and de Beaumont 1936, Hughes-Schrader 1975a) but not with . In the latter group, all hitherto studied species, which belong to the single family , show another very specific “parachute-like” sex bivalent in spermatocyte meiosis (Hughes-Schrader 1980, Takeuchi et al. 2002). In this case, the X and Y chromosomes form a pseudo-bivalent that is situated together with the autosomes on the equator of the spindle and segregates synchronously with them at the first meiotic anaphase. This unique meiotic sex chromosome configuration called Xyp (Smith 1950) is the well-known characteristic feature of the related order , and is encountered in almost all coleopteran families. Therefore Xyp is considered ancestral for beetles (Smith 1950), at least for the suborder (Petitpierre 1987).

The variety and distribution of sex chromosome systems in different orders of the class have been comprehensively reviewed by Blackman (1995). The X(0) system was shown to predominate in the lower orders and is considered as ancestral condition for several major groups and for as a whole. The XY systems when occur are all derived from an X(0) one. The sex chromosome systems seem to provide useful phylogenetic evidence. Within Holometabola orders, besides simple X(0) and XY, there are some peculiar systems, e.g., those involving female heterogamety (XY/XX or ZW/ZZ) shared by and , haplodiploid sex determination characteristic of , and some others. Of these, distance pairing of the X and Y chromosomes in spermatocyte meiosis and the parachute Xyp system are hypothesized to be synapomorphies respectively of the clade + and of the clade + (Blackman 1995, Takeuchi et al. 2002).