CONSERVATION OF ARABIAN GAZELLES - Nwrc.gov.sa

analysis. Systematic relationships based on chromosomal evolution can independently challenge
traditional taxonomic groupings based on morphological specializations.
In a growing number of mammalian species complexes, chromosomal variation between
populations exceeds apparent morphological variation. If chromosomal variation reproductively
isolates morphologically similar, or even identical, populations, they are referred to as cryptic, or
sibling, species (Mayr,1942). Reproductive isolation between two karyotypicaUy distinct, but
morphologically indistinguishable, forms of the antelope Madoqua kirki has been demonstrated by
Ryder et al. (1989) in captive animals; suggesting the existence of cryptic species within this taxon.
The extensive chromosomal divergence among captive G. soemmerringi is believed to be responsible
for their poor reproduction in captivity (Benirschke and Kumamoto, 1991). Unfortunately, the origin
of these gazelles is unknown, making it impossible to correlate the observed chromosomal divergence
with geographical populations. The important point to recognize, however, is that biological
diversity in the form of chromosomal divergence can only be identified through cytogenetic studies.
Biological Species Concept
Chromosomal variation has the potential to effect reproductive isolation between populations. The
Biological Species Concept stresses that populations be reproductively isolated and possess a genetic
programme effecting such isolation (Mayr, 1970). Although the degree to which chromosomal
rearrangements operate in reproductive isolation varies among groups of organisms (White, 1978) the
rate of speciation is clearly associated with the rate of chromosomal evolution in vertebrates (Fredga,
1977). In mammals, rates of speciation and chromosomal evolution are strongly correlated (Bush et
at., 1977). Only rearrangements which can potentially result in chromosomal underdominance
(inferiority of heterozygotes, in comparison to homozygotes, with respect to characters such as
fertility) can playa role in speciation processes (King, 1987). Chromosomal underdominance of
translocation heterozygotes has been demonstrated in humans (Speed, 1989) mice (Mus musculus; De
Boer and De long, 1989) and domestic cattle (Bos taurus; Dyrendahl and Gustavsson, 1979). The
1129 Robertsonian translocation that reduces fertility in cattle is of particular interest in considering
the fertility of gazelles with Robertsonian translocations. Poor reproduction of captive G.
soemmerringi is believed to be due to the "incompatibility" of 14 chromosomal cytotypes involving
three Robertsonian translocations (Benirschke et al., 1984; Benirschke and Kumamoto, 1991).
There is a deficiency of empirical data, particularly from wild populations, to test theoretical
aspects of mammalian chromosomal evolution (Baker et al., 1987). Many studies have reported
various translocations in domestic animals, and effects of these different translocations on
reproductive fitness varied intraspecifically and interspecifically (Long, 1988). It has been suggested
that the slow but positive increase of translocations in most domestic mammal species might be due
to a slight heterozygous advantage (heterosis). Another possibility is that the rearrangement in
heterozygotes is favoured through non-random assortment of chromosomes during meiotic
segregation. This has been documented in G. subgutturosa heterozygous for centric fusions
(Kings wood, 1992). King (1987) and Sites and Moritz (1987) contended chromosomal
rearrangements may affect reproductive fitness in one organism but not in another, and the study of
meiosis is one key to determining the influence of chromosomal variation upon the reproductive
process.
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