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.
Highlights
Spiders display a wide range of chromosome numbers and sex determining systems, and very commonly have multiple X chromosomes
Physocyclus mexicanus and Holocnemus pluchei spermatocytes were observed in metaphase and anaphase of meiosis I and meiosis II to determine chromosome number and sex determination mechanism
Our data show that Physocyclus mexicanus has a karyotype that is very similar to that of other species of Physocyclus
Summary
Spiders display a wide range of chromosome numbers and sex determining systems, and very commonly have multiple X chromosomes. The spider family Pholcidae currently consists of 84 genera and 1,111 species [3]. Of these species, fewer than 2% have published karyotype data [4,5,6,7,8,9,10,11,12,13,14,15,16]. While the majority of spiders have X1X20 (male)/X1X1X2X2 (female) sex determination, most studied pholcid species have X0 (male)/XX (female) sex determination, though X1X2 (male)/X1X1X2X2 (female) and X1X2Y (male)/X1X1X2X2 (female) sex determination systems have been observed [4,5,6,7,8,9,10,11,12,13,14,15,16]
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