Chromosomal Study on Selected Small Araneomorph Spiders from Brazil, Including the First Records in Palpimanidae and Theridiosomatidae (Araneae, Araneomorphae)

Abstract

Among the 50,474 spider species, only 849 have chromosomal data available in the literature. Fifty spider families remain unknown from a cytogenetic perspective. The aim of this study was to analyze chromosomally selected araneomorph spiders from Brazil, to contribute to the cytotaxonomy and chromosome evolution of this group. The karyotypes of 12 species belonging to families Corinnidae, Linyphiidae, Oonopidae, Palpimanidae, Theridiidae, Theridiosomatidae, Trachelidae, and Zodariidae were analyzed, including the first chromosomal record for the first two families. Specimens (with the abdomen content partially exposed by perforation) were subjected to colchicine, hypotonization, and fixation. In most cases, the total content of the abdomen was used to prepare slides by spreading of cell suspension and subsequent Giemsa staining. The results were as follows: Cinetomorpha simplex Simon, 1892 (Oonopidae) 2n♂ = 9, X0; Neotrops sp. and Neoxyphinus termitophilus (Oonopidae), 2n♂ = 7, X0; Otiothops birabeni (Palpimanidae); Agyneta sp. (Linyphiidae), 2n♂ = 24, X₁X₂0; Coleosoma floridanum, Thymoites sp.1 and Thymoites sp.2 (Theridiidae), 2n♂ = 22, X₁X₂0; Naatlo sp. (Theridiosomatidae), 2n♂ = 30, X₁X₂0; Orthobula sp. (Trachelidae) 2n♂ = 21, X0; Falconina sp. (Corinnidae), 2n♂ = 28, X₁X₂0; Epicratinus sp. (Zodariidae) 2n♂ = 42, X₁X₂0. The chromosomal morphology was determined for all the samples except for Oonopidae. Most species exhibited telocentric chromosomes, with the exception of Palpimanidae and Theridiosomatidae. The main findings: 1) support an hypothesis on ancestral karyotype of Zodariidae and Oonopidae; 2) reveal a relatively high chromosome number in Palpimanidae that supports an idea on relatively high ancestral chromosome number (2n♂ = 42) of entelegyne spiders (Palpimanoidea is sister group of entelegyne spiders); 3) show that the karyotype found in Theridiosomatidae is exceptional within the Araneoidea.

Keywords: Diploid number; Meiosis; Mitosis; Sex chromosome system.

Fig. 1.

Chromosomes of Oonopidae (Dysderoidea). Cinetomorpha simplex (A–C), Neotrops sp. (D–E) and Neoxyphinus termitophilus (F–G). A. Mitotic spermatogonial metaphase, 2n♂ = 9. B. Mitotic oogonial metaphase, 2n♀ = 10. C. Spermatocyte I, metaphase I showing four autosomal bivalents and a sex chromosome univalent (4II+X). D. Mitotic spermatogonial metaphase, 2n♂ = 7. E–F. Mitotic oogonial metaphase, 2n♀ = 8. G. Spermatocyte I, diplotene consisting of three autosomal bivalents and a sex chromosome univalent (3II+X). Scale bars = 5 µm.

Fig. 2.

Mitotic oogonial metaphase of Otiothops birabeni (Palpimanidae), 2n♀ = 36. Scale bar = 5 µm.

Fig. 3.

Chromosomes of Linyphiidae (A–D), Theridiidae (E–L) and Theridiosomatidae (Araneoidea). Agyneta sp. (A–D), Coleosoma floridanum (E–G), Thymoites sp.1 (H–J), Thymoites sp.2 (K–L) and Naatlo sp. (M-P): A: Mitotic spermatogonial metaphase composed of 24 telocentric chromosomes. B: Mitotic oogonial metaphase with 26 telocentric chromosomes. C: Spermatocyte I (early diplotene) with 11 autosomal bivalents and two positively heteropycnotic sex chromosome univalents arranged in parallel (11II+X₁X₂). D: Spermatocyte I (metaphase I) with 11 autosomal bivalents and two sex chromosome univalents (11II+X₁X₂). E, H: Mitotic spermatogonial metaphase, 2n♂ = 22 telocentric chromosomes. F, I, K: Mitotic oogonial metaphases, 2n♀ = 24 telocentric chromosomes. G, J, L: Spermatocyte I, metaphase I (G, J) and early diplotene (L) consisting of 10 autosomal bivalents and two sex chromosome univalents (10II+X₁X₂). Sex chromosomes are positively heteropycnotic at J and L. While they are associated at G, they pair in parallel in J. Mode of pairing is unclear at L. M: Mitotic spermatogonial metaphase, 2n♂ = 30. N: Spermatocyte I, metaphase I. Note 14 autosomal bivalents and two sex chromosome univalents (14II+X₁X₂). O: Metaphase II, n = 14. P: Metaphase II, n = 16 (14+X₁X₂). Sex chromosomes are positively heteropycnotic. Arrowhead = bivalent with two chiasmata. Scale bars = 5 µm.

Fig. 4.

Chromosomes of Corinnidae, Trachelidae, and Zodariidae (RTA Clade). Falconina sp. (Corinnidae) (A–E), Orthobula sp. (Trachelidae) (F–H) and Epicratinus sp. (Zodariidae) (I–L): A: Mitotic spermatogonial metaphase, 2n♂ = 28 telocentric chromosomes. B: Pachytene, 13II+X₁X₂. Sex chromosomes pair in parallel; they are positively heteropycnotic. C: Spermatocyte I, diplotene. Note 13 autosomal bivalents and two sex chromosome univalents (13II+X₁X₂). Sex chromosomes pair in parallel on the periphery of the plate. D: Metaphase II, n = 13 telocentric chromosomes. E. Metaphase II, n = 15 (13+X₁X₂), telocentric chromosomes. Sex chromosomes are associated and positively heteropycnotic. F. Mitotic spermatogonial metaphase, 2n♂ = 21. G: Mitotic oogonial metaphase, 2n♀ = 22 telocentric chromosomes. H: Spermatocyte I, diplotene consisting of 10 autosomal bivalents and a sex chromosome univalent (10II+X). Sex chromosome is positively heteropycnotic. I: Mitotic oogonial metaphase, 2n♀ = 44 telocentric chromosomes, except for the subtelocentric X₁. J: Spermatocyte I, diplotene composed of 20 autosomal bivalents and two sex chromosome univalents (20II+X₁X₂). Sex chromosomes are associated in parallel. K: Metaphase II, n = 20 telocentric chromosomes. L. Metaphase II, n = 22 (20+X₁X₂). Arrowhead = bivalent with two chiasmata. Scale bars = 5 µm.