Karyotype, sex determination, and meiotic chromosome behavior in two pholcid (Araneomorphae, Pholcidae) spiders: implications for karyotype evolution

Abstract

There are 1,111 species of pholcid spiders, of which less than 2% have published karyotypes. Our aim in this study was to determine the karyotypes and sex determination mechanisms of two species of pholcids: Physocyclus mexicanus (Banks, 1898) and Holocnemus pluchei (Scopoli, 1763), and to observe sex chromosome behavior during meiosis. We constructed karyotypes for P. mexicanus and H. pluchei using information from both living and fixed cells. We found that P. mexicanus has a chromosome number of 2n = 15 in males and 2n = 16 in females with X0-XX sex determination, like other members of the genus Physocyclus. H. pluchei has a chromosome number of 2n = 28 in males and 2n = 28 in females with XY-XX sex determination, which is substantially different from its closest relatives. These data contribute to our knowledge of the evolution of this large and geographically ubiquitous family, and are the first evidence of XY-XX sex determination in pholcids.

Figure 1. Karyotypes for Physocyclus mexicanus and Holocnemus pluchei and giemsa-stained meiosis I spermatocytes.

A. Haploid karyotype for Physocyclus mexicanus taken from fixed, giemsa-stained metaphase II spermatocyte. B. Haploid karyotype for Holocnemus pluchei taken from a fixed, giemsa-stained metaphase II spermatocyte (chromosomes 1–13 and Y chromosome) and a fixed, giemsa-stained anaphase I spermatocyte (X chromosome). C. Giemsa-stained metaphase I spermatocyte of Physocyclus mexicanus. X chromosome indicated by arrow. Bar = 10 µm. D. Giemsa-stained metaphase I spermatocyte of Holocnemus pluchei. X-Y bivalent indicated by arrow. Bar = 10 µm.

Figure 2. Metaphase I-Anaphase I in living Physocyclus mexicanus and Holocnemus pluchei spermatocytes.

A. Progression from metaphase I (0, 26, 29 min.) through anaphase I (36, 41, 47 min.) in Physocyclus mexicanus. A univalent chromosome remains near one spindle pole (arrow) through metaphase I and anaphase I. Bar = 10 µm. B. Progression from metaphase I (0, 4, 6 min.) through anaphase I (31, 37, 41 min.) in Holocnemus pluchei. X (arrow) and Y (arrowhead) chromosomes are located in the center of the spindle in metaphase (0, 4, 6 min.) and X and Y chromosomes separate from one another in anaphase (31, 37, 41 min.). Bar = 10 µm.

Figure 3. Micromanipulation of primary spermatocytes in Physocyclus mexicanus and Holocnemus pluchei.

A. In a Physocyclus mexicanus metaphase I spermatocyte (0 min.), a small micromanipulation needle was used to pull on the univalent chromosome near the spindle pole (2–4 min.), showing a single X chromosome (arrow). Bar = 10 µm. B. In a Holocnemus pluchei metaphase I spermatocyte (0 min.), a small micromanipulation needle was used to pull on the sex chromosomes in the middle of the spindle (0 min., arrow). Following the pulling, the chromosomes immediately return to their original position (1, 33 min.), indicating that chromosomes are connected to the spindle at both ends. X chromosome indicated by arrow, Y chromosome by arrowhead. Bar = 10 µm.