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Haplogyne araneomorphs are a diverse spider clade. Their karyotypes are usually predominated by biarmed (i.e., metacentric and submetacentric) chromosomes and have a specific sex chromosome system, X 1 X 2 Y. These features are probably ancestral for haplogynes. Nucleolus organizer regions ( NORs ) spread frequently from autosomes to sex chromosomes in these spiders. This study focuses on pholcids (  Pholcidae  ), a highly diverse haplogyne family. Despite considerable recent progress in pholcid cytogenetics, knowledge on many clades remains insufficient including the most species-rich pholcid genus,    Pholcus    Walckenaer, 1805. To characterize the karyotype differentiation of    Pholcus    in Europe, we compared karyotypes, sex chromosomes,  NORs , and male meiosis of seven species [   P.   alticeps    Spassky, 1932;    P.   creticus    Senglet, 1971;    P.   dentatus    Wunderlich, 1995;    P.   fuerteventurensis    Wunderlich, 1992;    P.   phalangioides    (Fuesslin, 1775);    P.   opilionoides    (Schrank, 1781);    P.   silvai    Wunderlich, 1995] representing the dominant species groups in this region. The species studied show several features ancestral for    Pholcus   , namely the 2n♂ = 25, the X 1 X 2 Y system, and a karyotype predominated by biarmed chromosomes. Most taxa have a large acrocentric  NOR -bearing pair, which evolved from a biarmed pair by a pericentric inversion. In some lineages, the acrocentric pair reverted to biarmed. Closely related species often differ in the morphology of some chromosome pairs, probably resulting from pericentric inversions and/or translocations. Such rearrangements have been implicated in the formation of reproductive barriers. While the X 1  and Y chromosomes retain their ancestral metacentric morphology, the X 2  chromosome shows a derived (acrocentric or subtelocentric) morphology. Pairing of this element is usually modified during male meiosis.  NOR  patterns are very diverse. The ancestral karyotype of    Pholcus    contained five or six terminal  NORs  including three X chromosome-linked loci. The number of  NORs  has been frequently reduced during evolution. In the Macaronesian clade, there is only a single  NOR -bearing pair. Sex chromosome-linked  NORs  are lost in Madeiran species and in    P.   creticus   . Our study revealed two cytotypes in the synanthropic species    P.   phalangioides    (Madeiran and Czech), which differ by their  NOR  pattern and chromosome morphology. In the Czech cytotype, the large acrocentric pair was transformed into a biarmed pair by pericentric inversion.

﻿Karyotype differentiation and male meiosis in European clades of the spider genus Pholcus ( Araneae , Pholcidae )

Karyotype differentiation and male meiosis in European clades of the spider genus Pholcus ( Araneae , Pholcidae )