Sexual reproduction is the way most life-forms procreate, or produce offspring. Each parent provides half an offspring's chromosomes. Over generations, this procreating shuffles the DNA cards, giving sexual reproducers genetic diversity. It is believed that this range of variation in genes can help organisms adapt more successfully to changing environments.

By contrast, asexual reproduction happens in some 70 vertebrate species—species with backbones—and in many less-complex organisms. Asexual species use "all the chromosomes they have" to solitarily produce offspring that are genetic clones, molecular biologist Peter Baumann says. The organisms are genetically identical. Because of that, they're more vulnerable: A disease or an environmental shift that kills one could kill all.

A Full Chromosome Count

But there's a twist in the case of the genus Aspidoscelis, the asexually reproducing whiptail lizard. Baumann and his colleagues have been studying this species at the United States' Stowers Institute for Medical Research in Kansas City, Missouri. The lizards are all female and parthenogenetic, meaning their eggs develop into embryos without fertilization.

But Baumann's team discovered something curious about the whiptail. Before the eggs form, the females' cells gain twice the usual number of chromosomes during meiosis (a stage of cell division during reproduction). This means that the eggs get a full chromosome count—a standard pair of chromosones derived from two sets of pairs. It also means they get genetic variety and breadth (known as heterozygosity) rivaling that of those from sexually reproducing lizards.

Baumann believes he knows why this occurs. At some time in the past, lizards of the genus Aspidoscelis had "a hybridization event," he explains.  Females of one species broke form and mated with males of another species. This gave whiptails robust heterozygosity, which has been preserved by the identical replication—essentially, cloning—that occurs in asexual reproduction. It's a genetic-diversity advantage that today's Aspidoscelis females still enjoy and replicate.

 

How an Asexual Lizard Procreates Alone

Without females, lizards in the Aspidoscelis genus, like this New Mexico Whiptail (Aspidoscelis neomexicana), reproduce asexually. Unlike other animals that produce this way, however, their DNA changes from generation to generation.

asexual reproduction
Noun

reproduction that requires only a single organism, without a union of individuals or gametes.

chromosome
Noun

strand of DNA and associated proteins in the nucleus of cells that carries the organism's genetic information.

clone
Noun

cell or group of cells that is genetically identical to its ancestor cell or group of cells.

DNA
Noun

(deoxyribonucleic acid) molecule in every living organism that contains specific genetic information on that organism.

generation
Noun

group in a species made up of members that are roughly the same age.

genetic
Adjective

having to do with genes, inherited characteristics or heredity.

genetic diversity
Noun

difference or variety of units of inheritance (genes) in a species.

heterozygous
Adjective

having two different alleles for the same gene

heterozygous
Adjective

having two different alleles for the same gene

hybrid
Noun

the end result of two different sources of input.

meiosis
Noun

type of cell division in which the daughter cells have half the number of chromosomes as the parent cell; also known as reduction division

meiosis
Noun

type of cell division in which the daughter cells have half the number of chromosomes as the parent cell; also known as reduction division

molecular biologist
Noun
scientist who studies the structure and activity of molecules essential to life.
parthenogenesis
Noun

method of reproduction in which the organism develops a female gamete without fertilization. Common among plants and invertebrates.

procreate
Verb

to breed.

reproduction
Noun

the process of breeding.

sexual reproduction
Noun

type of reproduction in which a sperm fertilizes an egg to form a zygote

vertebrate
Noun

organism with a backbone or spine.