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Proof is in the placenta

UC Riverside study suggests placentas can evolve in 750,000 years or less

Guppy-like fish help fill in the gaps in the evolution of complex organs

(December 20, 2002)

Placentas (left) that provision embryos (right) have evolved three times in <i>Peociliopsis</i> fish.  Photo credit:  David Reznick.

Placentas (left) that provision embryos (right) have evolved three times in Peociliopsis fish. Photo credit: David Reznick.

RIVERSIDE, Calif. -- Evolutionary biologists have long been intrigued by how natural selection -- the process in nature by which the organisms best suited to their environment are the ones most likely to survive and leave descendants -- gradually creates a complex organ such as the eye, heart, or kidney.

Now UC Riverside biologists, David Reznick and Mark Springer, along with Mariana Mateos, research associate at the University of Arizona, present in the journal Science a model system for studying the evolution of complex organs. They focus on the placenta (the organ that provides nutrients for the fetus and eliminates its waste products) in the fish genus, arguing that placentas serve as a good stand-in for complex organs whose histories have eluded evolutionary biologists.

The dilemma posed by complex adaptation, which are organs of extreme complexity that have evolved through the action of natural selection, is that these organs demand contributions from a large number of adaptations at individual genetic loci to function properly. Darwin addressed the difficulty of complex adaptations with his treatment of the evolution of the eye. "He had to use organisms from different classes," explained Reznick, "because there isn't a living group of related organisms that have all the steps for making an eye." The organisms in Darwin's model are, however, distantly related to one another.

Darwin proposed that complex eyes could have been formed with a succession of photosensitive organs, each a bit more complex than its predecessor and each favored by natural selection because of the advantages that the possessor received. Visualizing such a process would be easiest if steps in this sequence were preserved in closely related living organisms; but no such sequence exists for eyes because the intermediate stages have been lost through extinction.

Reznick and his colleagues studied guppy-like fish in the genus Poeciliopsis. They report that placentas have evolved independently three times in closely related Poeciliopsis species. Other species in the genus lack placentas, and some have partial maternal provisioning via tissues that may be precursors of placentas. "Thus the fish present the full trajectory of steps involved in the evolution of this organ," said Reznick. "It allows researchers to examine what's been added, or what has changed, and eventually identify the genes associated with the evolution of each trait."

The study by Reznick and colleagues first argues that the placenta is a complex organ, in the sense that it represents a composite of many adaptations and is controlled by many genes. "The origin of complex, novel organs plays a key role in evolution since they often define new categories of animals, such as the placenta for placental mammals," said Reznick. "They are also a source of controversy both within evolutionary biology and between evolutionary biology and the religious public. This is because their origin unfolds on a time scale considerably longer than human existence, so the process must be inferred indirectly."

In the Science paper, the researchers show that:
1) Fish in the genus Poeciliopsis have placentas in various stages of evolution, and
2) There are clusters of closely related species that either have highly evolved placentas, placentas in intermediate stages of evolution, or no placentas at all. These provide ideal material for studying how such complexity evolves.

The researchers then use the combination of molecular and geological data to yield estimates for how long it took the placenta to evolve in some lineages. Based on collected data, they find that the shortest time interval between a poeciliid species with a placenta and its last common ancestor without one was 750,000 years, suggesting that placentas can evolve in 750,000 years or less.

"This result demonstrates that complex organs can evolve rapidly, on the same scale as predicted by a theoretical estimate of 400,000 years for the evolution of the eye," said Springer.

Reznick has been collecting comparative life history data for around 15 years. For the study, he traveled around Latin America collecting the fish, going to museums to work with their collections, and then doing the appropriate dissections at UC Riverside. Several UC Riverside undergraduate students contributed to the dissections. Reznick also worked on live fish in his laboratory on campus.

The molecular work for the study was done by Mateos over the past two years. Springer did the phylogenetic work for the study. His statistical methods helped the researchers make inferences about how traits have evolved from the combination of DNA sequence data (collected by Mateos) and the descriptions of modes of reproduction (generated by Reznick).

The UC Riverside Department of Biology serves three main functions: undergraduate instruction, graduate education, and research in basic biology. The department conducts research and teaching in many areas of life science including cell biology, conservation biology, developmental biology, ecology, evolution, molecular biology, physiology, and population biology. The department is part of the College of Natural and Agricultural Sciences, a multi-departmental unit dedicated to instruction and basic research in the physical and life sciences, and also to 'mission-oriented' applied research in the agricultural sciences. The Biology major is a popular undergraduate major on the UC Riverside campus, with approximately 1000 students currently enrolled. Biology also provides much of the undergraduate instruction for majors in other life science departments and other science majors.

The University of California, Riverside ( is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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