so that when egg and sperm come together in fertilisation,
the diploid condition is restored. Obligate parthenogenetic
organisms can circumvent this problem by a doubling of
the genetic material prior to meiosis (a process dubbed
endoreduplication or endorecycling), so that the resultant
eggs then become diploid.
Parthenogenetic species typically arise through a
hybridisation between two closely-related sexually-
reproducing species, although the mechanism by which
this leads to parthenogenesis is not clear. A study
published in 2014 using a multi-locus dataset found that
this has been the case for all four parthenogenic species of
Leiolepis, the beautiful butterfly lizards of Southeast Asia.
The researchers found that all of the unisexual species
originated through hybridization events between
L. reevesii and L. guttata. In 2016, the discovery of a new
species of pleurodont iguanian was reported - the first
parthenogenic species in this group. Liolaemus parthenos
also arose through a hybridisation event.
The origins of parthenogenic species through hybridisa-
tion may help explain why these lineages persist, despite
obviously lacking the genetic variation produced by
sexual reproduction. Hybridisation between two closely
related sexual species means that these parthenogenetic
lizards contain a high degree of fixed heterozygosity and
hybrid vigour. Most parthenogenic species, and most
species in general, are diploid, with two sets of
chromosomes. However, some parthenogenic species are
triploid, with three sets of chromosomes, and tetraploid
specimens are not unknown. Triploid species arise when a
diploid parthenogenic species, created through a
hybridisation event, mates with a male of one of the
sexually-reproducing parental species. Polyploidy can
enhance hybrid vigour, and often confers increased size
and adaptability.
The hybrid origin of parthenogenic species means that
genetic variation within an individual is high, but because
of asexual reproduction, genetic variation between
individuals is virtually non-existent. However, there are
some parthenogenic species that have distinct clonal
lineages. Aspidoscelis dixoni is a parthenogenic unisexual
species of whiptail lizard that arose through hybridisation
between the sexual species A. tigris marmorata and
A. gularis septemvittata. However, A. dixoni has three
distinct colour classes across its range. Researchers
conducted reciprocal skin transplants to determine the
origins of these different classes; as all skin grafts were
‘
‘
Perhaps the best-known example of obligate partheno-
genesis in reptiles occurs amongst whiptail lizards native
to the southern United States and Central America and
belonging to the genera Cnemidophorus and Aspidoscelis
(previously part of the former). A study has shown that
triploid parthenogenic Aspidoscelis species employ
endoreduplication to produce gametes with an unreduced
chromosome content. The authors also concluded that
‘sister chromosome pairing provides a mechanism for the
maintenance of heterozygosity, which is critical for
offsetting the reduced fitness associated with the lack of
genetic diversity in parthenogenetic species.’
A female Eastern Diamond-backed
Rattlesnake once gave birth to nineteen
baby snakes after storing sperm for more
then five years!
Eastern Diamond-backed Rattlesnake (Crotalus
adamanteus). Image by Eric Isselee.