Neoromicia guineensis (Bocage, 1889)

Volleth, Marianne, Mayer, Frieder, Heller, Klaus-Gerhard, Müller, Stefan & Fahr, Jakob, 2023, Karyotype comparison of five African Vespertilionini species with comments on phylogenetic relationships and proposal of a new subtribe, Acta Chiropterologica 25 (1), pp. 35-52 : 40

Neoromicia guineensis (Bocage, 1889)

The single female studied showed a karyotype with a diploid chromosome number of 26 ( Fig. 2 ), including 12 pairs of autosomes: 10 large to medium-sized metacentric pairs, one small metacentric and one small acrocentric pair. The X chromosome was a medium-sized subtelocentric chromosome, bearing Myotis X homologous chromosomal material in the long arm only. The short arm of the N. guineensis X chromosome consisted of early replicating autosomal material (see below). The fundamental number of autosomal arms (FNa) of this species was therefore calculated as 46 arms arranged in 12 autosomal pairs plus two autosomal arms translocated to the X chromosomes, resulting in a total number of autosomal arms FNa = 48. The composition of the metacentric chromosomes revealed by G-banding was 1/2, 3/4, 5/6 and 16/17, as present in the vespertilionid basic karyotype, and 7/11, 8/9, 10/12, 13/18, 14/21, 15/19 and 20/22, resulting from Robertsonian fusions. The chromosomes 1/2, 11, 12, and 15, which exist in two different states in the Vespertilionidae were present in state II. A SC was found in chromosomal arm 15 close to the centromere, comprising a NOR as shown by silver-staining. Distally to the SC, a small heterochromatic segment could be detected in C-banded preparations.

In females of mammals, one of the two X chromosomes replicates early during S-phase, the second X replicates late, except for the very small pseudo-autosomal regions (the so-called inactive X, representing facultative heterochromatin). Therefore, by analysis of replication banding patterns, the X chromosomes in females can be distinguished from the autosomes. Analysis of the replication banding pattern in the female of N. guineensis revealed that only the long arm of the presumed X chromosome was late replicating ( Fig. 3 ). The G-banding pattern of the long arm of the X chromosome resembled in the proximal part the long arm and in the distal part the short arm of the X chromosome of state II of the vespertilionid basic karyotype. Strikingly, the G-band negative short arm of the guineensis X was composed of a small proximal C-positive heterochromatic segment and a distal euchromatic segment which replicated early also in the inactivated X ( Fig. 3 ). Therefore, this segment may be of autosomal origin. In order to clarify this issue and to search for homologous segments of the three smallest Myotis chromosomes (MMY23 to MMY25) in the karyotype of N. guineensis , we performed fluorescence in-situ hybridization (FISH). From the set of whole chromosome painting probes derived from M. myotis (MMY) the two smallest chromosomal pairs, number 24 and 25, could not be separated by flow sorting and were thus detected by one and the same painting probe. For this reason, an additional painting probe containing homologous sequences to the smallest chromosome of T. belangeri, TBE 30, for which homology to MMY24 was previously shown, was hybridized. The FISH results showed that the single acrocentric chromosome was composed of homologous sequences to MMY24 (and TBE30) in the proximal part and to MMY 23 in the distal part. In addition, the painting probe for MMY chromosomes 24+25 resulted in signals on the short arm of the subtelocentric X chromosome which is therefore homologous to MMY25. Subsequent application of the Myotis X-specific probe resulted in signals on the long arm of the X only, confirming that the guineensis X chromosome is indeed the product of an X-autosome translocation ( Fig. 3 ). Further FISH experiments with whole chromosome probes for Myotis chromosomes 16/17, 20, 21 and 22 were also in full agreement with the results of our comparative G-banding analysis.

Figures

FIG. 2. G-banded karyotype of a female N. guineensis, 2n = 26. Bold numbers for chromosomal arms indicate homology validated with painting probes from Myotis (MMY) or tree shrew (TBE30)

FIG. 3. X-autosome translocation in N. guineensis: The X chromosomes are shown after different staining procedures and FISH (G — G-banding, C — C-banding, R — replication banding procedure). Note the dark stained, early replicating distal part of the short arm in both X chromosomes, the early replicating (left homolog of the pair) and the late replicating (XL, right) long arms of the X chromosomes. FISH with the MMY X painting probe resulted in signals on the long arm of the N. guineensis X chromosome. FISH with a painting probe containing homologous sequences to Myotis chromosome 25 revealed signals on the short arm of the X