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Small pholcids (Araneae: Synspermiata) with big surprises: the lowest diploid number in spiders with monocentric chromosomes

Abstract Even though less than 2% of pholcid species have been karyotyped, previous studies documented a wide diversity of diploid numbers and sex chromosome systems. Here, we increase the number of native Brazilian cytogenetically investigated pholcid species from three to eight and discuss implications of chromosome evolution in this group. The species analyzed here share a X0/XX sex chromosome system and biarmed chromosomes, but vary in diploid numbers, i.e., 2n♂ = 17 in Mesabolivar spinulosus (Mello-Leitão, 1939) and Mesabolivar togatus (Keyserling, 1891), 2n♂ = 15 in Carapoia sp., and 2n♂ = 9 in Micropholcus piaui Huber, Carvalho & Benjamin, 2014 and Micropholcus ubajara Huber, Carvalho & Benjamin, 2014. Chromosomal data indicate that most Mesabolivar species share a 2n♂ = 17, X0, while Mesabolivar luteus shares with Carapoia sp. a 2n♂ = 15, X0. This lends further support to the idea that M. luteus is in fact misplaced and more closely related to Carapoia González-Sponga, 1998. The diploid number of the two Micropholcus species is the lowest reported so far for spiders with monocentric chromosomes. The 2n♂ = 9, differs strongly from the 2n♂ = 17 previously reported for Micropholcus fauroti (Simon, 1887). As the number of autosomes of M. piaui and M. ubajara is exactly half of that found in M. fauroti, we hypothesize that the reduction occurred by an “all or nothing” fusion event. The low diploid number observed in M. piaui and M. ubajara is the first morphological synapomorphy that would support the establishment of a new genus to allocate the New World Micropholcus species.