17
Chromosome Research 8: 17±25, 2000.
# 2000 Kluwer Academic Publishers. Printed in the Netherlands
Meiosis in holocentric chromosomes: orientation and segregation of an
autosome and sex chromosomes in Triatoma infestans (Heteroptera)
Ruben PeÂrez1 , Julio S. Rufas2 , Jose A. Suja2 , JesuÂs Page2,3 & Francisco Panzera1
SeccioÂn GeneÂtica Evolutiva, Facultad de Ciencias, Universidad de la RepuÂblica, IguÂa 4225, 11200
Montevideo, Uruguay; Fax: (5982) 5258629; E-mail: rperez@ fcien.edu.uy; 2 Unidad de BiologõÂa Celular,
Departamento de BiologõÂa, Edi®cio de Ciencias BioloÂgicas, Universidad AutoÂnoma de Madrid, 28049-Madrid,
Spain; 3 Current address: Programa de GeneÂtica Humana, Instituto de Ciencias BiomeÂdicas, Facultad de
Medicina, Universidad de Chile, Chile
Received 29 July 1999; received in revised form and accepted for publication by Pat Heslop-Harrison 10 November 1999
Key words: chromosome segregation, Hemiptera, holocentric chromosomes, meiosis, Triatoma infestans
Abstract
The meiotic behaviour of the X chromosome and one autosomal pair of the heteropteran Triatoma infestans was
analysed by means of C-banding plus DAPI staining. At ®rst metaphase, the X univalent is oriented with its long
axis parallel to the equatorial plate, which suggests a holocentric interaction with the spindle ®bres. After this
initial orientation, kinetic activity is restricted to one of both chromatid ends. The election of the active chromatid
end is random and it is independent of the end selected in the sister chromatid. At second metaphase, the X and
Y chromatids associate side by side forming a pseudobivalent. After that, the kinetic activity is again restricted to
either of both chromosomal ends in a random fashion.
At ®rst metaphase, the fourth autosomal bivalent shows two alternative random orientations depending on the
chromosome end showing kinetic activity (DAPI positive or opposite). At second metaphase, half bivalents are
oriented with their long axis parallel to the equatorial plate. Three different segregation patterns are observed.
The kinetic activity can be localised: (i) in the end with the DAPI signal (46.9%), (ii) in the opposite end (44.6%)
or (iii) in one DAPI-positive end in one chromatid and in the opposite end in the other one (8.5%). The existence
of the last pattern indicates that the same end can show kinetic activity during both meiotic divisions.
Our results provide new information on the comparative meiotic behaviour of autosomes and sex chromosomes in holocentric systems.
Introduction
Heteropteran insects have holocentric chromosomes
that are characterised by the presence of a diffuse or
non-localised centromere during mitosis (HughesSchrader & Schrader 1961). Microtubules attach to
the mitotic chromosome by means of a trilaminar
kinetochore, which occurs over a wide area of the
poleward chromatid surface (Buck 1967, Comings &
Okada 1972, GonzaÂlez-GarcõÂa et al. 1996a). This
holocentric interaction of microtubule ®bres leads to
a parallel segregation of sister chromatids during
mitotic anaphase (holokinetic movement).
Holocentric meiotic behaviour shows striking differences from mitosis. Kinetic activity in autosomes
and sex chromosomes is restricted to either of the
two chromatid ends during the ®rst and second
meiotic anaphase (Nokkala 1985, PeÂrez et al. 1997).
R. PeÂrez et al.
18
It used to be generally accepted that the autosomal
bivalents of Heteroptera show complete chiasma
terminalization, and that they dispose with their long
axes perpendicular to the equatorial plate (axial
orientation) (White 1973). However, recent studies in
the reduviid bug Triatoma infestans (PeÂrez et al.
1997) and the coreid Myrmus miriformis (Nokkala &
Nokkala 1997) do not support these generalisations.
Bivalents in these heteropteran species generally
show a single chiasma, which can occur in any part
of the chromosome and is not terminalised at ®rst
metaphase. Moreover, since either chromatid end can
show kinetic activity, two alternative orientations are
possible for a given bivalent at ®rst metaphase. In the
second division, the kinetic activity is also restricted
to either chromatid end. It has been suggested that
the end which is active in the ®rst division is inactive
in the second one (Nokkala 1985, PeÂrez et al. 1997).
Unlike the autosomal bivalents, the X and Y sex
chromosomes in heteropteran males are achiasmatic
and behave as univalents during the ®rst meiotic
division (Solari 1979). Reports on orientation of sex
chromosomes at the ®rst metaphase are con¯icting.
In Oncopeltus fasciatus (Wolfe & John 1965) and
Dysdercus intermedius (Ruthmann & Dahlberg 1976)
chromatids are joined only at one end and the X
chromosome disposed with its long axis parallel to
the polar axis. In contrast, in Graphosoma italicum,
the chromatids are joined all along their surface and
disposed with the long axes parallel to the equatorial
plate (GonzaÂlez-GarcõÂa et al. 1996b). At anaphase,
regardless of chromosome orientation, sister chromatids segregate polewards resulting in equational division. At ®rst telophase, one chromatid of the X and
one of the Y chromosome are associated end-to-end
to form a pseudobivalent which, during second metaphase, orientates with its long axis parallel to the
polar axis. In second anaphase these chromatids
segregate to opposite poles showing reductional division.
The absence of a localised centromere and, usually,
suitable chromosome markers have hindered the
study of holocentric behaviour. In this sense, variations of the traditional banding methods, such as Cbanding plus DAPI staining, which has been used
successfully in other organisms (Bella & GonsaÂlvez
1994) offer new possibilities to overcome these dif®culties. In this paper we analysed the meiotic behaviour of an autosome and sex chromosomes in T.
infestans using this ¯uorescent banding technique.
Materials and methods
Triatoma infestans (Hemiptera±Reduviidae±Triatominae) is the main vector of Chagas' disease in South
America. We analysed ®ve adult males of T. infestans
collected from natural populations in Uruguay. Testes
were removed, ®xed in an ethanol±glacial acetic acid
mixture (3 : 1) and stored at ÿ208. Squashes were
made in a drop of 50% acetic acid. The C-banding
technique was carried out on air-dried squashes following the technique described by PeÂrez et al.
(1997). For ¯uorochrome application, C-banded preparations were stained for 20 min with DAPI (2 ìg/
ml), instead of Giemsa, rinsed in tap water, air-dried
and mounted in Vectashield (Vector Laboratories).
Results
Chromosome complement. Giemsa and DAPI
staining
The chromosome complement of T. infestans consists
of 20 autosomes plus two sex chromosomes (XY in
males, XX in females) (Schreiber & Pellegrino
1950). Conventional C-banding (with Giemsa staining) shows heterochromatic blocks in the three large
autosomal pairs (Figure 1A). In the individuals analysed, C-bands are located at both ends of chromosomes 1 and 2 and at only one end of chromosome 3.
The sex chromosomes are middle sized, the Y
chromosome being almost totally C-heterochromatic
and larger than the X chromosome, which does not
show any Giemsa C-positive region (Figure 1C).
All C-heterochromatic regions showed high bright
¯uorescence when DAPI staining was used after Cbanding treatment (Figure 1B). Moreover, all the
individuals analysed show a DAPI-positive signal in
the X chromosome, which is totally euchromatic after
conventional C-banding. Two individuals also present
a DAPI-positive signal at one end of the fourth
bivalent pair (arrows in Figure 1B).
Meiotic behaviour of sex chromosomes
At ®rst metaphase, the sex chromosomes are disposed at the equatorial plate, occupying the centre of
a ring formed by the ten autosomal bivalents.
Although they appear together, they do not form a
true bivalent and do not have any physical connec-
Meiosis in holocentric chromosomes
19
Figure 1. Mitotic prometaphase and meiotic metaphase of Triatoma infestans. C-banding revealed by Giemsa (A and C) and DAPI (B, D
and E) staining. (A, B) Spermatogonial mitotic prometaphase. (C, D) First metaphase. (E) Second metaphase. (A) Only three pairs of
autosomes and the Y chromosome present C-blocks. (B) A fourth autosomal pair (arrows) and the X chromosome show DAPI-positive
regions at only one end. These regions are not observed with Giemsa staining. (C) At ®rst metaphase, the sex chromosomes are disposed at
the equatorial plate, occupying the centre of a ring formed by the ten autosomal bivalents. (D) The X univalent is oriented with its long
axis perpendicular to the polar one. (E) The X±Y pseudobivalent show a `side-by-side' disposition.
tions. The X univalent, with their chromatids joined
all along their surface, is oriented with its long axis
perpendicular to the spindle axis (Figures 1D & 3a).
After this initial orientation, the sex chromatids segregate and kinetic activity is restricted to one of the
two chromosomal ends (Figure 2). It is not possible
to analyse the orientation of the Y chromosome
because it appears totally ¯uorescent and its chromatids cannot be distinguished.
The X chromosome presents three different types
of segregation during ®rst meiotic division that we
have named X1 , X2 and X3 . In segregation type X1 ,
chromatid separation begins at the ends bearing the
positive DAPI signal; thus the kinetic activity is
restricted to the same end in both chromatids (Figure
2A & 3b). Type X2 corresponds to those cases in
which chromatid separation initiates at the two chromatid ends lacking the DAPI signal, which again
indicates that kinetic activity is restricted to the same
end in both chromatids (Figure 2B & 3c). Finally, in
segregation type X3 , chromatid separation begins at
the end with the ¯uorescent signal in one of the
chromatids, whereas in its sister chromatid it begins
at the end without the DAPI signal (Figures 2C &
3d). In this case, the kinetic activity is present at
opposite ends of each chromatid, which is clearly
observed in ®rst anaphase (Figure 2D). The following
frequencies of each type of segregation were quanti®ed in 184 cells of two individuals: 28.3% (type X1 ),
24.5% (type X2 ) and 47.2% (type X3 ). If we assume
that chromatid ends have the same probability of
showing kinetic activity, and also that this behaviour
is independent for each chromatid, the distribution of
types X1 , X2 and X3 should be 1 : 1 : 2. Statistical
analysis does not show a signi®cant deviation when
the observed and expected values are compared
(÷ 2 ˆ 1.08; p , 0.58).
The second meiotic division occurs after the ®rst
20
R. PeÂrez et al.
Figure 2. Different types of segregation of the X chromosome in T. infestans during the ®rst (A±D) and second division (E, F). (A) Type
X1 , chromatid separation begins at the ends bearing the positive DAPI signal. (B) Type X2 , chromatid separation initiates at both chromatid
ends lacking the DAPI signal. (C) Type X3 , chromatid separation begins at the end with the ¯uorescent signal in one of the chromatids,
whereas in the sister chromatid it begins at the end without the DAPI signal. (D) First anaphase corresponding to type X3 . (E) The
pseudobivalent is joined by the end with the DAPI signal (Type I). (F) The pseudobivalent is joined by the end without the DAPI signal
(Type II).
telophase almost without a resting stage. Initially, the
X and Y chromosomes show relics of the telophase
disposition, i.e. they are side by side (Figures 1E &
3e). After that, the chromatids form a pseudobivalent
localised in the centre of the ring constituted by the
autosomes (Figure 2E). During the second meiotic
anaphase, kinetic activity is also restricted to the
chromosomal ends (Figure 2F). Two different segregations of the X chromosome that form the pseudobivalent have been observed in 314 cells of ®ve
Meiosis in holocentric chromosomes
21
Figure 3. Selected X chromosomes showing the segregational behaviour during the ®rst (a±d) and second division (e±g). (a) The X
univalent is oriented with its long axis perpendicular to the spindle axis. (b) Type X1 : chromatid separation begins at the ends bearing the
positive DAPI signal. (c) Type X2 : chromatid separation initiates at both chromatid ends lacking the DAPI signal. (d) Type X3 : chromatid
separation begins at the end with the ¯uorescent signal in one of the chromatids, whereas in the sister chromatid it begins at the end
without the DAPI signal. (e) `Side-by-side' disposition of the X±Y pseudobivalent. (f) The pseudobivalent is joined by the end with the
DAPI signal (Type I). (g) The pseudobivalent is joined by the end without the DAPI signal (Type II).
individuals undergoing the second meiotic division.
In 51% of the cases (Type I), the kinetic activity is
located at the end lacking the DAPI signal (Figures
2E & 3f), whereas, in the remaining 49% (Type II),
the kinetic activity occurs at the end with the DAPIpositive signal (Figures 2F & 3g). The observed
frequencies indicate that the choice of one or the
other end is random since no signi®cant differences
with a 1 : 1 distribution were found (÷ 2 ˆ 0.11;
p , 0.73).
Meiotic behaviour in the fourth autosomal pair
The presence in two individuals of a positive DAPI
signal at only one chromosomal end of the fourth
autosome allows us to determine the kinetic behaviour of each chromatid end during both meiotic
divisions. At ®rst metaphase, this bivalent shows two
alternative segregations that we have named AA1 and
AA2 . In these orientations kinetic activity is either
located at both ends without DAPI signals (Type
AA1 ; Figures 4A & 5a) or at the ends with DAPI
signals in both chromosomes (Type AA2 ; Figures 4B
& 5b). In order to test whether the choice of ends
(DAPI positive or the opposite one) is a random
process, we scored both bivalent con®gurations. The
frequencies observed in 373 cells in ®rst metaphase
(Type AA1 , 53.1%; Type AA2 , 46.9%) show that the
choice is a random process (÷ 2 ˆ 1.42, p , 0.23).
In the second metaphase, the fourth half bivalent is
oriented parallel to the equatorial plate with its
chromatids joined (Figures 4C & 5c). After this
initial orientation, three different types of segregation
in autosomes (Type A) can be observed. Kinetic
activity can be found:
Type A1 ± in both chromatid ends with the
¯uorescent signal (46.9%) (Figures 4D & 5d).
Type A2 ± in both chromatid ends lacking the
¯uorescent signal (44.6%) (Figures 4E & 5e).
Type A3 ± in the ¯uorescent end in one of the
chromatids, whereas in the other chromatid the
kinetic activity is at the end lacking the
¯uorescent signal (8.5%) (Figures 4F & 5f).
The observed frequencies, scored over 729 cells,
are highly signi®cantly different from the 1 : 1 : 2
expected from random segregation ( p , 0.001).
R. PeÂrez et al.
22
Figure 4. Segregational behaviour of the fourth autosomal pair of T. infestans during the ®rst (A, B) and second (C±F) metaphase. (A) The
kinetic activity is found in the end lacking the DAPI signal (Type AA1 ). (B) The kinetic activity is found at the DAPI-positive end (Type
AA2 ). (C) The half bivalent is oriented with its long axis parallel to the polar one. (D) Type A1 ± both ends with the DAPI signal show
kinetic activity. (E) Type A2 ± both ends lacking the DAPI signal show kinetic activity. (F) Type A3 ± kinetic activity is found in the
DAPI-positive end of one of the chromatids, whereas in the other chromatid the kinetic activity is at the end lacking the ¯uorescent signal.
Discussion
There are three aspects of interest in relation to the
meiotic behaviour of holocentric chromosomes in
Heteroptera:
(i) The orientation of autosomes and sex chromosomes during the ®rst and second division.
(ii) The rules that govern the choice of the
kinetically active ends.
(iii) Whether the kinetically active ends in the
®rst division are also active in the second
one or, alternatively, there is an inversion of
kinetic activity between both divisions.
These events cannot be analysed in most organisms
since holocentric chromosomes are heavily condensed at metaphase and lack suitable chromosome
markers. In this paper, we show that the C-banding
procedure plus DAPI staining can detect ¯uorescent
signals. These signals can be used as markers to
analyse the behaviour of chromosomes that do not
have heterochromatic blocks.
Sex chromosome behaviour
The X chromosome of T. infestans orientates with its
long axes parallel to the equatorial plate in ®rst
metaphase (Figures 1D & 3A). This orientation was
also observed in the coreid Protenor (Schrader 1935)
and in the pentatomid Graphosoma italicum (GonzaÂlez-GarcõÂa et al. 1996b). We suggest that this could
Meiosis in holocentric chromosomes
23
Figure 5. Selected autosomes showing the segregational behaviour during the ®rst (a and b) and second division (c±f). (a) Kinetic activity
is restricted to the ends without the DAPI signal (Type AA1 ). (b) Kinetic activity is restricted to the ends with the DAPI signal (Type AA2 ).
(c) The half bivalent is oriented with its long axis parallel to the polar one. (d) Kinetic activity is restricted to the end of the chromatid with
the DAPI signal (Type A1 ). (e) Kinetic activity is restricted to the end of the chromatid lacking the DAPI signal (Type A2 ). (f) Kinetic
activity in one chromatid is at the end bearing the DAPI signal but in the other chromatid the kinetic activity is at the opposite end (Type
A3 ).
be the usual disposition of sex chromosomes in most
Heteroptera, detectable only when chromosome markers are available. The alternative disposition described in Oncopeltus fasciatus (Wolfe & John 1965)
and Dysdercus intermedius (Ruthmann & Dahlberg
1976) is presumably a later stage, previous to the
anaphase segregation.
The disposition of the X chromosome observed
during ®rst metaphase suggests that there is an initial
holocentric interaction between microtubules and the
whole surface of the chromatids. Although this disposition resembles mitotic behaviour, it must be
remembered that it is produced without the presence
of a kinetochore as occurs in mitosis (Buck 1967).
The onset of anaphase coincides with a change in the
interaction of microtubules from the holocentric to an
end-restricted contact. These results suggest that
microtubule interaction during meiosis has, at least,
two steps: ®rst, an initial holocentric interaction
which determines the stabilisation of the chromosomes, and second, the end-restricted interaction
which is involved in the segregation of the chromatids.
In reference to the anaphase segregation of the sex
chromosomes, similar to the observations in G. italicum, the choice of the active chromosomal end in
each chromatid is a random process and it is not
in¯uenced by the selection occurring in the other
chromatid.
In second metaphase, the `side-by-side' disposition
of the X and Y chromatids seems to be a relic of ®rst
anaphase segregation (Figures 1E & 3e) since the
resting stage between the ®rst and second meiotic
divisions is very short. At second anaphase, kinetic
24
activity is again detected at the chromosomal ends
(Figure 2F). Several authors have suggested that
kinetic activity in the second anaphase is inverted if
compared with the one observed in the ®rst division
(Wolfe & John 1965, Nokkala 1985, Albertson &
Thomson 1993, PeÂrez et al. 1997). This means that
the end that shows kinetic activity in the ®rst division
is inactive in the second one. This seems quite
reasonable if we consider that the end that shows
kinetic activity in the ®rst anaphase is thereafter
involved in the association of the pseudobivalent in
late telophase and second metaphase. The end-to-end
association of the X and Y chromatids could inhibit
in some way their ability to show kinetic activity in
the second division. We cannot prove this hypothesis
because the observed 1 : 1 distribution of frequencies
®ts both models of segregation, i.e. inversion or not.
Orientation and segregation of autosomes
At ®rst meiotic division of T. infestans, PeÂrez et al.
(1997) established that, in chromosomal pairs with
large C-blocks, the euchromatic end is selected more
frequently than the heterochromatic end. However, in
this paper, we show that, in bivalents lacking Cblocks (but with C-banding-DAPI positive signals),
the selection of one or the other end seems to be a
random process. The presence of a large heterochromatic block (like the one presented in autosome
three) seems to produce a shift of kinetic activity to
the opposite (euchromatic) end. This effect could be
produced directly; i.e. heterochromatin is less ®t for
kinetic activity, or indirectly, through the effect of the
heterochromatic block over other factors, for example, chiasma position. It has been proposed that
chiasma proximity could produce an inhibition of
kinetic activity (Camacho et al. 1985). In triatomines,
both the heterochromatic block and the chiasma
position seem to affect this choice (data not shown).
However, they are not the only factors involved in
orientation and segregation of chromosomes. A given
bivalent, with a chiasma in a given place, is able to
show kinetic activity in the heterochromatic end in
one cell and in the euchromatic one in another cell.
This fact indicates that other factors, beside the
heterochromatic block and chiasma position, in¯uence the election of kinetically active end. More
studies are necessary to clarify the mechanism/s that
in¯uence the choice of those ends with kinetic activity in holocentric chromosomes of Heteroptera.
R. PeÂrez et al.
At second meiotic division, the initial orientation
of half bivalents (parallel to the equatorial plate) is
followed by a disposition where one of the ends of
the chromatids face the pole while the other end
maintains chromatid association (Figures 4D±F). As
suggested for the X chromosome, this situation could
also involve some shift from a broad spindle ®bre
attachment to a terminal location of microtubules in
anaphase.
In our previous paper (PeÂrez et al. 1997), using
exclusively C-banding, we detected that kinetic activity was restricted to the same chromosomal end in
both chromatids. In this paper, and probably due to
the better resolution afforded by DAPI staining, we
could detect a third possibility: kinetic activity is
found in opposite ends of both chromatids (Type A3 )
(Figures 4F & 5f). This type of segregation is observed with a low frequency (8.5%) and it supports
an initial holocentric interaction of half bivalents at
second metaphase (Figures 1E & 3e). After the
chromosome is holokinetically stabilised, the ®bres
have to be restricted or concentrated to opposite ends
to explain the migration of Type A3 at anaphase.
In relation to kinetic activity, during the ®rst division, only two ends (with the DAPI signal or without
it) of the bivalent are active at the same time. The
existence, during second division, of Type A3 implies
that the same chromosomal end is active during both
divisions. At least in these few cases there is no
inversion of kinetic activity as reported by other
authors (Nokkala 1985, PeÂrez et al. 1997).
Comparative behaviour of sex chromosomes and
autosomes
Our results suggest that the segregational behaviour
of sex chromosomes in the ®rst metaphase and half
bivalents in the second one share a common mechanism. This behaviour may be determined by the number of chromatids of the chromosome to be
segregated. Thus, the organization of the sex univalents during the ®rst meiotic metaphase may be compared to that of half bivalents during the second
metaphase. In both cases the chromosomes orient
with their long axes parallel to the equatorial plate
(compare Figures 1D & 4C) to later segregate with
kinetic activity restricted to the chromosomal end.
It would be of outstanding interest to study autosomal univalents in triatomines produced by asynap-
Meiosis in holocentric chromosomes
sis or desynapsis, and compare their behaviour with
half bivalent and sex chromosomes.
Acknowledgements
This work is partially supported by grants from
CONICYT-Uruguay (Project 2034), PM 95/0038
(Project PB98-0107, DGES-Spain), AVINA Foundation (Switzerland) and the European CommunityLatin America Triatominae (ECLAT) research network (Project ERBIC 18-CT98-0366). We are also
grateful to the Agencia EspanÄola de CooperacioÂn
Internacional (AECI).
References
Albertson DG, Thomson JN (1993) Segregation of holocentric
chromosomes at meiosis in the nematode Caenorhabditis elegans. Chromosome Res 1: 15±26.
Bella JL, GosaÂlvez J (1994) Banding human chromosomes using a
combined C-banding-¯uorocrome staining technique. Biotech
Histochem 69: 243±248.
Buck RC (1967) Mitosis and meiosis in Rhodnius prolixus: the ®ne
structure of the spindle and diffuse kinetochore. J Ultrastruct
Res 18: 489±501.
Camacho JPM, Belda J, Cabrero J (1985) Meiotic behaviour of the
holocentric chromosomes of Nezara viridula (Insecta, Heteroptera) analysed by C-banding and silver impregnation. Can J
Genet Cytol 27: 491±497.
Comings DE, Okada TA (1972) Holocentric chromosomes in
Oncopeltus: kinetochore plates are present in mitosis but absent
in meiosis. Chromosoma 37: 177±192.
25
GonzaÂlez-GarcõÂa JM, Benavente R, Rufas JS (1996a) Cytochemical and immunocytochemical characterization of kinetochores
in the holocentric chromosomes of Graphosoma italicum. Eur J
Cell Biol 70: 352±360.
GonzaÂlez-GarcõÂa JM, Antonio C, Suja JA, Rufas JS (1996b).
Meiosis in holocentric chromosomes: kinetic activity is randomly restricted to the chromatid ends of sex univalents in
Graphosoma italicum (Heteroptera). Chromosome Res 4: 124±
132.
Hughes-Schrader S, Schrader F (1961) The kinetochore of the
Hemiptera. Chromosoma 12: 327±350.
Nokkala S (1985) Restriction of kinetic activity of holokinetic
chromosomes in meiotic cells and its structural basis. Hereditas
102: 85±88.
Nokkala S, Nokkala C (1997) The absence of chiasma terminalization and inverted meiosis in males and females of Myrmus
miriformis Fn. (Corizidae, Heteroptera). Heredity 78: 561±566.
PeÂrez R, Panzera F, Page J, Suja JA, Rufas JS (1997) Meiotic
behaviour of holocentric chromosomes: orientation and segregation of autosomes in Triatoma infestans (Hemiptera: Reduviidae). Chromosome Res 5: 47±56.
Ruthmann A, Dahlberg R (1976) Pairing and segregation of the sex
chromosomes in X1 X2 -males of Dysdercus intermedius with a
note on the kinetic organization of heteropteran chromosomes.
Chromosoma 54: 89±97.
Schrader F (1935) Notes on the mitotic behaviour of long chromosomes. Cytologia 6: 422±430.
Schreiber G, Pellegrino J (1950) Eteropicnosi di autosomi come
possible meccanismo di speciazione (Ricerche citologiche su
alcuni Emitteri neotropici). Sci Genet 3: 215±226.
Solari AJ (1979) Autosomal synaptonemal complexes and sex
chromosomes without axes in Triatoma infestans (Reduviidae,
Hemiptera). Chromosoma 72: 225±240.
White MJD (1973) Animal Cytology and Evolution, 3rd edn. Cambridge: Cambridge University Press, London.
Wolfe SL, John B (1965) The organization and ultrastructure of
male meiotic chromosomes in Oncopeltus fasciatus. Chromosoma 17: 85±103.