Hypervariability of Nucleolus Organizer Regions in Bengal Slow Lorises, <b><i>Nycticebus bengalensis</i></b> (Primates, Lorisidae)

Abstract

Slow lorises are a cryptic species complex, and thus genetic markers are needed to identify distinct evolutionary lineages or species. We examined the nucleolus organizer regions (NORs) of Bengal slow lorises (Nycticebus bengalensis) using FISH with 18S rDNA (rDNA-FISH) and silver nitrate staining (Ag-NOR stain). Ten individuals of the putatively single species N. bengalensis showed higher variability in localization than 3 other congeners, though their overall karyotypes were similar. The rDNA-FISH analysis detected a total of 18 loci, in contrast to previous studies of other slow loris species that revealed far fewer (6-10) loci. Eight of the 18 loci detected in the present analysis were found to be semi-stable localizations at 4 different chromosomes, while 10 were found to be unstable localizations at 5 other chromosomes. The semi-stable locations showed occasional presence/absence of variations for rDNA-FISH, and unstable locations were polymorphic among individuals, contributing to the higher variability of NORs in this taxon. We hypothesize that the larger numbers of rDNA loci found in N. bengalensis were introduced by genomic dispersion through ectopic recombination in association with terminal regions including rDNA. Such differences are potentially very powerful chromosomal markers to be used in species identification and conservation.

Evolutionary insight on localization of 18S, 28S rDNA genes on homologous chromosomes in Primates genomes

“…Indeed, among Primates , we found the rDNA loci on HSA synteny 3 on Platyrrhini species S. sciureus , C. jacchus , C. goeldii and in gibbons H. lar and S. syndactilus . In addition, data from literature on the Prosimian Nycticebus bengalensis Lacepede, 1800, ( Baicharoen et al 2016 ) show that rDNA loci exist on human synteny 3. Furthermore, we identified rDNA loci on HSA synteny 22 in the Prosimian representative L. catta .…”

“…The topology of rDNA loci is widely used as marker for comparative cytogenetic studies and to explore evolutionary relationships, since such loci often show species-specific patterns ( Srikulnath et al 2009 , 2010, Cazaux et al 2011 , Bulatova and Pavlova 2016 ). Furthermore, the variation in number and topology of rDNA genes has been shown at inter- and intra-species levels, explained as consequence of chromosomal rearrangements, ectopic recombination through association of rDNA with other chromosomal segments during meiotic division or transposition events ( Hirai et al 1996 , Eickbush and Eickbush 2007 , Baicharoen et al 2016 ).…”

“…rDNA distribution especially of the 18S and 28S loci has been investigated in many species of Primates either by FISH (Henderson et al 1974a,b, 1976 , 1977 , 1979 , Hirai et al 1999 , 2007 , Guillén et al 2004 , Baicharoen et al 2016 ) or silver staining ( Tantravahi et al 1976 , Bedard et al 1978 , Masters et al 1987 , Nagamachi et al 1992 , Hirai et al 2007 , Tanomtong et al 2009 ), including Homo sapiens Linnaeus, 1758. In humans, NORs have been identified on the secondary constriction of five pairs of acrocentric chromosomes: 13, 14, 15, 21 and 22 ( Henderson et al 1972 , Tantravahi et al 1976 ).…”

“…Furthermore, exchanges among rDNA genes on non-homologous chromosomes (Arnheim et al 1980) and a multiple topologies of rDNA sites with species-specific variations ( Hirai et al 1999 ) have been shown in Hominoidea . Later, intra-species polymorphisms have also been described in Primates such as Pan troglodytes (Blumenbach, 1775) ( Guillén et al 2004 ), Hylobates lar (Linnaeus, 1771), ( Tanomtong et al 2009 ) and Nycticebus bengalensis (Geoffroy, 1812) ( Baicharoen et al 2016 ), possibly related to unequal crossing over or to transcriptional inactivation by methylation of NORs.…”

Chromosome Dynamics Regulating Genomic Dispersion and Alteration of Nucleolus Organizer Regions (NORs)

“…The association of different chromosomes leads to increased variability around NORs, and the proximity of the rDNA arrays and then predisposes them to recombination at the associated terminal regions. If there is an error in the 'molecular effect' at terminal associations (i.e., breakdown of the system of avoidance among distinct genome components), recombination will bring about new intra-specific variations that result from such atypical relationships among rDNA and other repeat arrays [44,59].…”