Biology Reference
In-Depth Information
Putative primordial defensin gene
I II
Tandem duplication with
accumulation of nucleotide changes
Ancestral defensin 6
Ancestral defensin 5
I
II
I
II
Homologous unequal crossover
I
II
I
II
I
II
I
II
Reciprocal
product lost
I
II
Ancestral
defensin 6
Ancestral defensins
1, 3 & 4
Ancestral
defensin 5
I
II
I
II
I
II
Acquisition of upstream exon
with hematopoietic promoter
Recent duplication
Human
defensin 6
Human
d
efensins 1 & 3
Human
defensin 4
Human
defensin 5
I
II
I'
II'
III'
I'
II'
III'
I
II
Figure 9.5.
Model for the involvement of an homologous unequal crossover in the
evolution of the human defensin gene family (after Bevins
et al
., 1996).
Shaded enclosed boxes denote exons, with exons I and II representing the two exons of
the epithelial defensin genes
DEFA5
and
DEFA6
. Exon I
is the characteristic upstream
exon of hematopoietic defensins whilst exons II
are homologous to exons I and
II of the epithelial defensins. Solid boxes denote regions of striking conservation between
all defensins. Small arrows denote the locations of the transcription start sites.
and III
The
SCYA18
(17q11.2) gene, which encodes a member of the small inducible
cytokine family, appears to have been generated by the fusion of two macrophage
inflammatory protein-1
-like (
SCYA3
) genes with subsequent deletion and selec-
tive use of exons (Tasaki
et al.
, 1999). Since there are several related genes (
SCYA3
,
SCYA3L1
and
SCYA3L2
) in the vicinity of
SCYA18
, the authors suggested that
the
SCYA3
gene might represent a 'hot spring' that continually generates new
genes by duplication and fusion.
The human immunoglobulin
Fc receptor IIC (
FCGR2C
; 1q23) gene is also
thought to have resulted from an unequal crossover, this time between the
