Abstract
The American dragonfly genus Orthemis Hagen, 1861 is mainly found in the Neotropical region. Seven of 28 taxonomically described species have been reported from Argentina. Chromosome studies performed on this genus showed a wide variation in chromosome number and a high frequency of the neoXY chromosomal sex-determination system, although the sexual pair was not observed in all cases. This work analyzes the spermatogenesis of Orthemisdiscolor (Burmeister, 1839), O.nodiplaga Karsch, 1891 and O.ambinigra Calvert, 1909 in individuals from the provinces of Misiones and Buenos Aires, Argentina. Orthemisdiscolor has 2n=23, n=11+X and one larger bivalent. Orthemisnodiplaga exhibits the largest chromosome number of the order, 2n=41, n=20+X and small chromosomes. Orthemisambinigra shows a reduced complement, 2n=12, n=5+neo-XY, large-sized chromosomes, and a homomorphic sex bivalent. Fusions and fragmentations are the main evolutionary mechanisms in Odonata, as well as in other organisms with holokinetic chromosomes. Orthemisnodiplaga would have originated by nine autosomal fragmentations from the ancestral karyotype of the genus (2n=22A+X in males). We argue that the diploid number 23 in Orthemis has a secondary origin from the ancestral karyotype of family Libellulidae (2n=25). The complement of O.ambinigra would have arisen from five autosomal fusions and the insertion of the X chromosome into a fused autosome. C-banding and DAPI/CMA3 staining allowed the identification of the sexual bivalent, which revealed the presence of constitutive heterochromatin. We propose that the chromosome with intermediate C-staining intensity and three medial heterochromatic regions corresponds to the neo-Y and that the neo-system of this species has an ancient evolutionary origin. Moreover, we discuss on the mechanisms involved in the karyotypic evolution of this genus, the characteristics of the neo sex-determining systems and the patterns of heterochromatin distribution, quantity and base pair richness.
Highlights
Libellulidae is characterized by having a modal number 2n=25 (n=12+X) in males, an XX/X0 chromosomal sex-determination system and chromosomes that decrease gradually in size, with the X chromosome being one of the smaller in the complement (Mola et al 1999; Mola 2007; Kuznetsova and Golub 2020)
Chromosome studies performed on this genus have revealed two particular characteristics: first, a wide variation in chromosome number, ranging from 2n=7 with two bivalents and a trivalent in meiosis (n=2II+1III) in Orthemis levis Calvert, 1906 to 2n=41 (n=20+X0) in O. nodiplaga, with no species having the characteristic modal number 25 of Libellulidae; and, second, a high frequency of neoXYsex-determining systems (Cumming 1964; Cruden 1968; Kiauta and Boyes 1972; Ferreira et al 1979; Kiauta 1979; Souza Bueno 1982; Agopian and Mola 1984a)
Kiauta and Boyes (1972) assumed that the typical number of the genus would be of primary origin, as opposed of being derived from the modal number of the family
Summary
Libellulidae is characterized by having a modal number 2n=25 (n=12+X) in males, an XX/X0 chromosomal sex-determination system and chromosomes that decrease gradually in size, with the X chromosome being one of the smaller in the complement (Mola et al 1999; Mola 2007; Kuznetsova and Golub 2020). Chromosome studies performed on this genus have revealed two particular characteristics: first, a wide variation in chromosome number, ranging from 2n=7 with two bivalents and a trivalent in meiosis (n=2II+1III) in Orthemis levis Calvert, 1906 to 2n=41 (n=20+X0) in O. nodiplaga, with no species having the characteristic modal number 25 of Libellulidae; and, second, a high frequency of neoXYsex-determining systems (Cumming 1964; Cruden 1968; Kiauta and Boyes 1972; Ferreira et al 1979; Kiauta 1979; Souza Bueno 1982; Agopian and Mola 1984a). About half of the species in the order with this neoXY system have a homomorphic sex bivalent in males, including the species of Orthemis (Oksala 1943; Cumming 1964; Kiauta 1971, 1972, 1979; Ferreira et al 1979; Souza Bueno 1982; Mola 1996; Perepelov et al 1998; Perepelov and Bugrov 2002)
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