If God exists and has a gender she’s definitely a woman,” says biologist Rodrigo Marques Lima dos Santos, excited when he see what lizards – or rather female lizards – manage to do.
Several species of lizards exhibit surprising ways of reproducing. Females generate their young asexually, without the participation of any male. They are independent, but not radical: in some species, if a male passes close by they allow copulation and may be fertilized. Reproductive autonomy reaches such a point that in some species there are only females, which reproduce in an asexual manner known as parthenogenesis, which, it now seems, is more flexible than was previouly thought.
Biologists from the University of São Paulo (USP), from the Federal University of São Paulo (Unifesp) and from the National Laboratory of Biosciences (LNBio), when studying different aspects of parthenogenesis, concluded that alterations in a gene known as c-mos might enable the transformation of female reproductive cells (ova) into an embryo, even without spermatozoa.
Rodrigo Santos followed the trail after this mechanism in his PhD, while he was studying the teiidae lizards, a group that includes species that range from 10 cm long to the tejus, which grows up to 1.5 meters in length. Without expecting it, he began to see mutations in the c-mos gene in parthenogenetic species. In 2008 he began to work with Andréa Balan, from LNBio, to model the forms of the protein produced by c-mos in lizards and snakes. Together they identified mutations in one of the four active sites (interaction points) of the protein, thus reinforcing the initial hypotheses.
The c-mos gene produces a protein that blocks the end of cell division of the ovum until the spermatozoon arrives. When the male sexual cell fertilizes the ovum it deactivates the protein, cell division ends and an embryo is formed. The hypothesis of the researchers is that when the c-mos undergoes alterations it does not function correctly and may make the ovum continue dividing, even without a spermatozoon. They believe that defects in this gene could attenuate the blocking of the division of the ovum and allow other stimuli, like hormones, to reactivate cell division.
If this work progresses it might throw light on one of the mechanisms of parthenogenesis. Today, little is known about how species of lizard that are capable of reproducing asexually came about, and even less about how they acquired and maintain this skill. According to the most widely accepted hypothesis, parthenogenetic snakes and lizards may be the result of crossing between close species.
Leposoma percarinatum, one of the species found in Brazil, is showing the reach of this genetic maze. The lizards of this species, recognized as parthenogenetic in 1952, are at most 5 centimeter long and live among leaves in the forests of an extensive region that stretches from Venezuela to the north of Mato Grosso State and from the Andes to the east of Pará. One hypothesis, presented in the 1970s, suggests that L. percarinatum is the result of the crossing of two different species, Leposoma guianense and L. parietale, which are found in the humid forests of South America.
Katia Pellegrino, from Unifesp, and Miguel Rodrigues, from USP, came across an unusual situation: the females of Leposoma percarinatum were practically the same appearance-wise but had an astonishing difference from the genetic point of view. Some females, the diploids, had 44 chromosomes (two equal pairs of 22 chromosomes) in each cell, while the triploids had 66 chromosomes (three pairs of 22).
“Within what had been supposed to be the same species there are two different strains, which will allow us to reconstruct their history and the mechanisms of their origin,” Katia concluded. She believes that the triploid variety must have arisen from another hybridization event between the diploid form of L. percarinatum and L. osvaldoi, since L. guianense does not occur so far to the south of Brazil.
Sometimes creatures turn up that do away with all explanations. From a trip to the archipelago of Anavilhanas on the Negro River, Rodrigues brought back specimens of Leposoma guianense, some individuals which were revealed to belong to a new clone of Leposoma percarinatum and others that were different to the point of representing a new species, which was named Leposoma ferrerai. They were all diploids living in the same area.
The whiptail lizard, or Cnemidophorus nativo, one of the few species of Brazilian lizard that is exclusively parthenogenetic (and that is also threatened with extinction) is merely diploid, according to the analyses of Santos. Found in the forests in the north of Espírito Santo and the south of Bahia, these animals belong to a sister family of the Leposoma, but can grow to 30 cm long. According to Santos, other parthenogenetic species that live in the Amazon, such as the Cnemidophorus lemniscatus and Gymnophthalmus underwoodi, seem to have mixed diploid and triploid populations.
The biologists are working with the possibility that parthenogenesis does not just form clones of the mother, but also allows some genetic variation, although smaller than with sexual reproduction, through recombination in chromosomes of the ovum. “A recent study showed that a snake generated an albino offspring by parthenogenesis, indicating that there is, indeed, genetic recombination even in asexual reproduction,” argues Santos. “The spontaneous origin of parthenogenesis, an alternative hypothesis to the hybrid theory, cannot be discounted in Leposoma and Cnemidophorus, since this mechanism has already been suggested for specimens of Gymnophthalmus underwoodi from Roraima,” adds Katia.
An indispensable embrace
Santos believes that Cnemidophorus nativo may behave similarly to lizards of the genus Aspidoscelis. Found in desert regions in Asia and North America, Aspidoscelis only begins to form embryos after being embraced, which biologists call pseudo-copulation. When one of them detects the touch or grating from another it should activate the release of hormones that unblock the c-mos, the biologists from USP believe.
“For some parthenogenetic species of the genus Aspidosceles to reproduce,” Santos comments, “copulation between females is obligatory.” David Crews and Jon Sakata, from the University of Texas, showed in 2000 that embracing females had a reverse hormonal cycle, one with high levels of estrogen, the most abundant hormone in females, and the other with high levels of testosterone, produced more intensely by males.
In 2011, researchers from the University of Kansas managed to induce hybridization and confirm that sexual reproduction may form a parthenogenetic species by crossing two species of Aspidosceles. Making a parthenogenetic female reproduce in the laboratory, alone or through being ‘cuddled’ by another female, however, remains one of the dreams of the biologists.
Of the 5,634 species of lizard already identified, some 40 are parthenogenetic and they generally live in tropical forest regions or the desert climates of Asia or Oceania. “Reproduction by parthenogenesis results in less genetic variability than with sexual reproduction, but may be an adaptive survival response to extreme environments,” comments Yatiyo Yassuda, a geneticist specializing in lizard genetics who is monitoring the study on the possible origins of parthenogenesis.
In the 1980s, Yatiyo faced a similar problem and at great cost managed to convince other geneticists that lizards of the genus Tropidurus had a sexual differentiation; the males had a different chromosome from the females, but it was so small that it was almost imperceptible. Many species of lizard have the same set of chromosomes and are differentiated sexually by unknown genes or temperature variations while they are developing; if the temperature is higher it may favor the development of male embryos in some species, or females in others.
Yatiyo’s room, where Santos talks about his work, has some paintings of flowers, some figurative, others abstract. “This was the beginning. I’ve already painted more than 300 pictures since I retired,” she says, imagining the sun she intended to paint on the following day, a Saturday. “But I still come here every day.”
“Meiotic parthenogenesis is a form of natural cloning similar in part to induced cloning for the reproduction of animals that are of commercial interest,” he observes. In 2004, researchers from the Paulista State University (Unesp) in Jaboticabal indicated that ethanol and the chemical strontium can induce cell division in the ova of cows, functioning like an external stimulus, analogous to the male sexual cell.
Similarly, the eggs of experimental animals, like Dolly the sheep, only developed after they had received an electric shock, which must deactivate the c-mos. Santos believes that if parthenogenesis could be regulated it could help in livestock farming or in the conservation of wild species at risk of extinction. “Mammals have mechanisms that avoid parthenogenesis, like imprinting,” he observes. Another application would be medical, since mutations in this gene might make the eggs divide uncontrollably, giving rise to tumors.
If they make progress, maybe the biologists will find new answers to two basic questions of biology. The first: what is sex for? The other: what is the advantage of sexual reproduction? According to Santos, sexual reproduction requires the meeting of two organisms to form offspring, while in parthenogenesis just one organism is sufficient to generate another. Genetic variability resulting from sexual reproduction is not always beneficial to species, argue the biologists.
“Sexual reproduction is better for environments in transformation, with a high risk of predation and disease, but it is bad in stable environments and healthy populations, because a well-adapted individual can produce offspring which do not adapt well ,” he says. “In stable environments, clonal reproduction, as carried out with the breeding of animals and plants, result in offspring with high yields, is the most frequently indicated.”
1. The systematics and evolution of neotropical herpetofauna (nº 2003/10335-8); Modality Thematic Project; Coordinator Miguel Trefaut Rodrigues – USP; Investment R$ 975,589.35.
2. Genetic diversity in unisexual and bisexual species of Cnemidophorus of the Ocellifer (Teiinae) group and structural characterization of the Mos protein in the Squamata (nº 2008/56444-6); Modality Post-doctoral Studies Grant; Coordinator Rodrigo Marques Lima dos Santos – USP; Investment R$ 277,872.66.
3. Cytogenetic and molecular studies in micro-teiidae lizards (Squamata, Gymnophthalmidae), with an emphasis on species of the genus Leposoma from the Amazon and Atlantic rainforests (nº 1998/05289-7); Modality Post-doctoral Studies Grant; Coordinator Katia Cristina Machado Pellegrino – Unifesp; Investment R$ 37,720.00.