Biology Reference
In-Depth Information
Constructing a medium-density genetic linkage map of microsatellite
markers of
Astatotilapia burtoni
, Sanetra et al. (2009) showed the presence of
25 linkage groups spanning 1249.3 cM of the genome size (~ 950 Mb) with
an average marker spacing of 6.12 cM. The seven
Hox
clusters,
ParaHoxC1
and two paralogues of
Pdgfrβ
have also been mapped to different linkage
groups; their fi ndings support the hypothesis of teleost-specifi c genome
duplication, with the formation of an astonishingly large number of species
in a record-breaking time in cichlids.
Amazingly, among the Malawi cichlids investigated by Ser et al. (2009),
seven species have XX/XY system and another three possess ZZ/ZW system
(Table 8). Evidences from several species strongly suggest the presence
of additional sex determining loci and cryptic epistatic interactions. For
instance, the known two sex determining XX/XY and ZZ/ZW systems
co-exist and segregate within single families of
Metriaclima pyrsonotus.
When both dominant sex determinant loci W and Y are present in a single
individual (Table 8), the ZZ/ZW system dominates over the XX/XY
system.
Table 8.
Known linkage groups and sex chromosome systems in selected Malawi cichlids
(compiled from Ser et al., 2009)
Species
Sex chromosome system
Linkage group (LG)
Aulonocara baenschi
XX/XY
7
Pseudotropheus polit
XX/XY
7
Metriaclima barlowi
XX/XY
7
M. mbenjii
XX/XY
7
M. phaeos
XX/XY
7
M. zebra
XX/XY
7
M. pyrosonotus
Family Mlpy A002, A017
Family Rmpy 018, 021
XX/XY
ZW/XY
7
5
M. calloinos
ZW/XY
5
M. fainzilberi
ZW/XY
5
Labeotropheus trewavasae
ZW/XY
5
Uniquely, the platyfi sh
Xiphophorus
maculatus
has three different,
genetically defi ned sex chromosomes X, Y and W (Kallman, 1984). Possible
female genotypes are XX, XW and YW, whereas males harbor the XY or
YY constellations. Unlike in mammals, YY male and ZZ female fi shes are
viable and fertile (Pandian, 2011). WW females have not been recorded from
natural populations but can be generated in the laboratory and are viable as
well (Schultheis et al., 2009). Sex determining mechanisms among the many
species of
Xiphophorus
are quite diverse and have been well established in 12
species (Walter et al., 2006).
X. maculatus
Jp 163A is a female homogametic
