TRABAJOS CIENTÍFICOS
Chromosome complement and meiosis of Holmbergiana weyenberghii (Opiliones: Sclerosomatidae: Gagrellinae) from Argentina
Complemento cromosómico y meiosis de Holmbergiana weyenberghii (Opiliones: Sclerosomatidae: Gagrellinae) de Argentina
Rodríguez Gil, Sergio G. and Liliana M. Mola
Laboratorio de Citogenética y Evolución, Departamento de Ecología Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires. Intendente Güiraldes y Costanera Norte, 1428 Ciudad Universitaria. Ciudad Autónoma de Buenos Aires, Argentina; e-mail: rodrigil@ege.fcen.uba.ar, limola@ege.fcen.uba.ar
ABSTRACT. The cytogenetical analysis of the harvestman Holmbergiana weyenberghii (Holmberg) (Eupnoi, Sclerosomatidae, Gagrellinae) from Argentina is reported for the first time. The complement of males is composed of 18 chromosomes. In meiosis there are nine homomorphic bivalents: one large, five medium-sized and three small. The chromosome number of H. weyenberghii is within the range of diploid numbers of the subfamily Gagrellinae Thorell, which shows the lowest chromosome numbers among the sclerosomatids.
KEY WORDS. Meiotic behavior; Holmbergiana weyenberghii; Opiliones; Gagrellinae.
RESUMEN. Se analiza citogenéticamente, por primera vez, una especie de opilión proveniente de Argentina: Holmbergiana weyenberghii (Holmberg) (Eupnoi, Sclerosomatidae, Gagrellinae). Los machos tienen un complemento cromosómico compuesto por 18 cromosomas. En meiosis, hay nueve bivalentes homomórficos: uno mayor, cinco medianos y tres menores. El número cromosómico de H. weyenberghii se encuentra dentro del rango de números diploides de los Gagrellinae Thorell; esta subfamilia presenta los números cromosómicos más bajos de Sclerosomatidae.
PALABRAS CLAVE. Comportamiento meiótico; Holmbergiana weyenberghii; Opiliones; Gagrellinae.
Recibido: 2-XII-2009;
Aceptado: 8-VI-2010
INTRODUCTION
The Order Opiliones Sundevall,
comprises four suborders (Cyphophthalmi
Simon, Eupnoi Hansen & Sørensen, Dyspnoi
Hansen & Sørensen, and Laniatores Thorell)
with more than 6.000 taxonomically
described species (Shultz & Regier, 2001;
Giribet et al., 2002; Hallan, 2009). The family
Sclerosomatidae Simon, of the suborder
Eupnoi, includes four subfamilies from the
Old and New World: Gagrellinae Thorell,
Leiobuninae Banks, Sclerosomatinae Simon,
and Gyinae Silhavy (Acosta & Maury, 1998;
Hallan, 2009). The former two subfamilies are
represented in Argentina; Gagrellinae, with about 1,100 species distributed worldwide,
has the largest number of taxonomically
described species of the family, and among
these only 13 occur in Argentina (Acosta & Maury, 1998; Hallan, 2009). Holmbergiana
weyenberghii Holmberg is an Argentine
species typical of the Mesopotamian region,
which is also found in the Pampean region
(Acosta & Maury, 1998).
The cytogenetic analysis of harvestmen
has been performed in less than 1,5% of
taxonomically described species. These
showed a wide range of diploid numbers,
from 10 in Systenocentrus japonicus Hirst,
Paraumbogrella pumilio (Karsch), some
chromosome races of Gagrellula ferruginea
(Loman) (Eupnoi) and some populations
of Sabacon makinoi Suzuki (Dyspnoi), to
92-109 in Goniosoma spelaeum (Mello-Leitão) (Laniatores) (Oliveira et al., 2006;
Tsurusaki, 2007). Most species have nondistinguishable
sex chromosomes, the XX/XY
system (female/male) was found in several
species of Gagrellinae and Leiobuninae
(Sclerosomatidae, Eupnoi) and in Sabacon
makinoi (Sabaconidae, Dyspnoi), while
the ZW/ZZ system (female/male) was only
observed in Mitopus morio (Fabricius) and
Odiellus aspersus (Karsch) (Phalangiidae,
Eupnoi) (Tsurusaki, 1985; Tsurusaki & Cokendolpher, 1990; Tsurusaki, 2007). The
subfamilies of Sclerosomatidae analysed
were Gagrellinae (7 species), Leiobuninae
(28 species) and Sclerosomatinae (3 species);
these belong to the Old World, except for
five species of Leiobuninae occurring in USA
(Tsurusaki, 1985, 2006, 2007).
This is the first cytogenetic study of an
Argentine opilionid species, Holmbergiana
weyenberghii (Sclerosomatidae,Gagrellinae),
with a description of some spermatogonial
chromosome characteristics and meiotic
development.
MATERIAL AND METHODS
The specimens of Holmbergiana
weyenberghii were collected in Buenos
Aires City (34º 36' 30" S - 58º 22' 23" W)
(five males, March 1997) and in the Horco
Molle Ecological Reserve in San Javier Hill,
Province of Tucumán (26º 46' 22" S - 62º 21' 13" W) (one male, September 1997),
Argentina.
The specimens were transported alive to
the laboratory and determined taxonomically
by the late Dr. E. Maury (Museo Argentino de
Ciencias Naturales "Bernardino Rivadavia").
The harvestmen were anesthetised with
ether prior to dissection of the testes, which
were fixed in 3:1 (absolute ethylic alcohol:
glacial acetic acid), preserved in 70%
ethanol and stored in the refrigerator until
use. Preparations were made by squashing
the material in acetic hematoxylin.
RESULTS
The males of Holmbergiana weyenberghii showed a complement composed of 18 biarmed chromosomes. At spermatogonial late prophase the chromosomes show heterochromatic regions of different size in the telomeric regions and, to a lesser extent, in interstitial regions (Fig. 1). At early prophase I there is no positive heteropycnotic body (Figs. 2-3). Nine homomorphic bivalents are seen from diplotene onwards. One bivalent is conspicuously larger, five are medium-sized and decrease gradually in size and three are small (Figs. 4-6). The larger and the medium-sized bivalents may show two terminal chiasmata or a single terminal or sub-terminal chiasma; while the small bivalents only have one terminal chiasma (Figs. 4-6). All the chromosomes migrate synchronously at anaphase I, and at metaphase II nine chromosomes are seen (Fig. 7). No differences were observed between locations.
Figs. 1-7. Mitosis and meiosis in Holmbergiana weyenberghii (2n=18, n=9); 1, Late spermatogonial
prophase; 2, Pachytene; 3, Late pachytene; 4-5, Diakinesis; 6, Prometaphase I; 7, Metaphase II. The
arrowheads point to the larger pair. The arrows point to the bivalents with two chiasmata. Scale bar =
10 μm.
DISCUSSION
Thirty-eight species of Sclerosomatidae
have been cytogenetically studied, and
only seven of these belong to the subfamily
Gagrellinae (Tsurusaki, 1985, 2006, 2007),
which shows the lowest chromosome
numbers. This subfamily has a diploid chromosome number varying from 10 in
Systenocentrus japonicus, Paraumbogrella
pumilio and some chromosome races
of Gagrellula ferruginea, to 22 in some
populations of Gagrellopsis nodulifera Sato & Suzuki and other chromosome races of
Gagrellula ferruginea (Tsurusaki et al., 1991;
Gorlov & Tsurusaki, 2000a; Tsurusaki, 2007).
All available data on the Gagrellinae were
exclusively obtained from material collected
in Japan except for Gagrella unicolor (Roewer) (sub Melanopa unicolor) from
India (Sharma & Dutta, 1959). Cytogenetic
studies of South American harvestmen
were performed in only eight species of the
family Gonyleptidae Sundevall (Laniatores)
from Brazil, which showed the highest
chromosome numbers of the order (Oliveira
et al., 2006; Tsurusaki, 2007). The study
of Holmbergiana weyenberghii represents
the first contribution to the cytogenetic
knowledge of Eupnoi in South America.
In Gagrellinae the sex determination
system XY/XX (male/female) has been
reported in four species: Paraumbogrella
pumilio, Psathyropus tenuipes L. Koch
(sub Metagagrella tenuipes), Gagrellopsis
nodulifera, and Gagrella unicolor (Sharma
and Dutta, 1959; Tsurusaki, 1982, 1993;
Tsurusaki et al., 1991). In Gagrella unicolor male meiosis the sex chromosomes are seen
as one or two heteropycnotic bodies at early
prophase I and as a heteromorphic pair from
diplotene to metaphase I; furthermore they
precede the migration of autosomes at both
meiotic anaphases (Sharma and Dutta, 1959).
In Holmbergiana weyenberghii neither of
these features was seen; nevertheless, it is
not possible to rule out the presence of sex
chromosomes due to the lack of suitable
metaphase plates to allow a detailed
karyotypic analysis.
It is interesting to point out that the
presence of two chiasmata in a variable
number of bivalents seems to be frequent
in Gagrellinae species, since besides H.
weyenberghii, it has been reported in at
least other four species, Paraumbogrella
pumilio, Psathyropus tenuipes, Gagrellopsis
nodulifera, and Gagrella unicolor (Sharma & Dutta, 1959; Tsurusaki, 1982, 1993;
Tsurusaki et al., 1991; Gorlov & Tsurusaki,
2000b).
The diploid number of H. weyenberghii (2n=18) falls within the range of the
chromosome numbers of the subfamily; this
diploid number is also found in Gagrella
unicolor and Psathyropus tenuipes. In the
latter species the chromosome number
increases within and between populations
due to the presence of a variable number of
supernumerary chromosomes, from one to
18 B-chromosomes (Tsurusaki, 1993; Gorlov & Tsurusaki, 2000b). Other chromosome
number variations in populations
of Gagrellinae result from structural
rearrangements giving rise to chromosome
races as in Gagrellula ferruginea (2n=10-22) and Gagrellopsis nodulifera (2n=14-22).
Several hybrid zones have been reported
for the latter species in Japan, where
neighbouring populations with different
chromosome numbers come in contact
and produce heterozygotic karyotypes with
different chromosome numbers (Gorlov & Tsurusaki, 2000a; Tsurusaki, 2007).
Despite the limited cytogenetic studies
on harvestmen, particular features have
been reported for the order, such as different
chromosomal sex-determination systems,
B-chromosomes, polyploidy, chromosome
races and hybrid zones in Old World species
(Tsurusaki, 2007). Further research is needed
to clarify the population cytogenetic features
of New World species, representing only 21% of the chromosomally analysed species.
Acknowledgements
The present study has been performed with grants from the Buenos Aires University (UBA) (X317, X178) and CONICET (PIP 5927, PIP 0342) to Dr. Lidia Poggio and Liliana Mola. The authors thank the late Dr. Alba G. Papeschi for her collaboration in chromosomal studies.
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