Reproductive biology of rotifers

Bdelloid Rotifer Reproduction

Rotifers are, perhaps, one of the most interesting phyla of microscopic pseudocoelomate animals on the face of the planet. Each tiny animal is approximately 0.1-0.5 mm in length and are found in freshwater around the globe, and in saltwater in certain areas. The name rotifer is derived from Latin meaning 'wheel bearer', and was spawned due to their some of their unique characteristics ("Rotifer," 2004).

Interestingly rotifers in the class of Bdelloidea have done what no other animal has yet accomplished. They have been able to successfully reproduce asexually, while evolving to meet the changing environmental conditions. Although other animals reproduce asexually, they are typically not evolutionarily successful. The questions that are posed in this paper are how do the bdelloids reproduce, and how have they been able to evolve successfully with asexual reproduction.

Bdelloid Rotifer Reproduction

Introduction:

Rotifers are, perhaps, one of the most interesting phyla of microscopic pseudocoelomate animals on the face of the planet. Each tiny animal is approximately 0.1-0.5 mm in length and are found in freshwater around the globe, and in saltwater in certain areas. The name rotifer is derived from Latin meaning 'wheel bearer', and was spawned due to their some of their unique characteristics ("Rotifer," 2004).

The corona of the rotifer is composed of several ciliated tufts positioned around the mouth. When these tufts are in motion, they resemble a wheel, hence the name. It is the motion of these tufts that sweep food into the mouth of the rotifer and is then chewed by the mastax that contains tiny jaws. In addition to feeding, the motion of these tufts provides locomotion, pulling the rotifer through the water (Ryan, 2004). (See Images 1-4)

Despite this characteristic commonalities, rotifers come in a variety of different shapes. Some are box-like in shape. Others resemble a worm-like structure. Some rotifers live inside tubes, while others are planktonic ("Rotifer," 2004). However, the most interesting similarity these diverse

Image 1: Image 2:

The cilia wave back and forth in a The cilia are arranged in double sequential pattern giving the rows around the corona forming illusion of rotation. The trochal discs.

Image 3: Image 4:

The red eye spots are located Bdelloid rotifers are divided into just in front of the mastax. three main segments: head, trunk and foot.

Source: Webster, 2003 microscopic creatures have is their ability to reproduce by parthenogenesis.

Major Questions Regarding Bdelloid Rotifer Reproduction:

Clearly one of the most pressing questions regarding rotifers is how exactly they are able to reproduce in a sexless environment, as the "vast majority of multicellulsr creatures have sex to reproduce" (Ryan, 2004). In addition, one would question how they've been able to survive for so long without sexual reproduction.

Typically, organisms that do not utilize sexual reproduction do not evolve, as do their sexual brethren. For this reason, they most often become extinct over the course of just a few hundred thousand years. However, rotifers are the exception. They not only have survived for more than 50 million years, but also have evolved into approximately 370 species (Ryan 2004).

Observations or Evidence that have Bearing on the Questions Raised:

In most rotifer species, the males are reduced to finite numbers or are even completely absent. With the latter, reproduction occurs by parthenogenesis, which means "the growth and development of an embryo or seed without fertilization by a male" ("Parthenogenesis," 2004). All of the rotifers in which males have been replaced, continue to evolve despite their asexual reproduction techniques (Ryan, 2004).

This is contrary to what scientists have long held to be true. Out of nearly 2 million species on the planet Earth, only 2,000 are asexual. Of these 2,000, fossil evidence shows that these species are quite young on the evolutionary scale, all save rotifers. Asexual species simply do not last. (as cited in Milius, 2000). That is, except for the Bdelloidea rotifer.

It is the rotifers of the Class Bdelloidea that are significantly different from the rotifers belonging to the other three classes of the Rotifer phylum. Bdelloidea are completely asexual, where the other three are sexual (Welch & Meselson, 2000).

Bdelloidea rotifers are "the largest metazoan taxon in which males, hermaphrodites, and meiosis are unknown" (Welch & Meselson, 2000). As such, Welch & Meselson conducted genetic tests to test this theory. Their test was based on the anticipation that without sexual reproduction, genomes would no longer contain pairs of similar haplotypes. Instead, they surmised, that highly divergent descendant of formerly allelic nucleotide sequences would be found. They found that in four different species, pairs of similar sequences were lacking. It was this evidence that showed that two ancient lineages were represented, from a time millions of years ago when sexual reproduction ceased to exist for the rotifers (Welch & Meselson, 2000).

These researchers further identified the uniqueness of the rotifer asexuality. They noted that Few species of animals or plants reproduce only asexually -- and those that do seldom make up an. entire genus, let alone a taxon of higher rank. These observations have been taken to mean that the loss of sexual reproduction is a dead end in evolution, leading to early extinction. Against this generalization, the entire Class Bdelloidea of the Phylum Rotifera stands out as an apparently radical exception, an "evolutionary scandal" (Welch & Meselson, 2000).

The father of microscopy, Antonie van Leeuwenhoek, studied rotifers more than 300 years ago. Rotifers may be tiny, but they have differentiated parts, including ovaries. Yet, it is these ovaries that are so different from other species.

When an animal uses sexual reproduction, gonads are used to split the genome in half to make eggs and sperm. However, a rotifer ovary creates eggs that contain the full genome of the creature. "Eggs are produced from oocytes in well-differentiated ovaries by two mitotic divisions with no chromosome synapsis and no reduction in chromosome number, each oocyte giving rise to one egg and two polar bodies" (Welch & Meselson, 2000). There is no need for sperm for these eggs to mature into adults (Milius, 2003).

In a sense, the parent rotifer clones itself. Its genes are copied for the next generation and the next. However, small errors in the copying process seem to equate to noticeably different genetic copies after multiple generations. Ryan (2004) compares this process as being similar to faxing a printed page back and forth several times and comparing it to the original document. "David Mark Welch, of the Marine Biological Laboratory in Woods Hole, Massachusetts, and colleagues found just such divergent gene copies, and no identical gene pairs, in every bdelloid gene they examined" (Ryan, 2004).

However, there is still the possibility that rotifers are involved in some unknown form of sexual reproduction. "A living insect species once thought not to have male forms, for example, turned out to include males that were perfectly functional but very, very tiny" (Milius, 2003). Extreme inbreeding may be the cause for the almost identical gene pairs found in the population. This has been the case in other species, and sent scientists on a quest to discover a truly asexual species that has the ability to evolve (Ryan, 2004).

A second possibility exists. The lack of allele pairs in individual genomes could result from bdelloids being "haploid females of sexually reproducing species that have an evanescent or unrecognized diploid form" (Welch & Meselson, 2000).

If organism are able to reproduce effectively utilizing asexual reproduction then what is sex for? Sexual reproduction is only half as efficient as asexual reproduction, therefore efficiency is not a benefit. Some scientists believe sexual reproduction is a way for organisms to purge harmful mutations from the genome, or allows the organism to evolve as quickly as rapidly evolving pathogens (Ryan, 2004), or simply to cope with environmental changes as they occur (Milius, 2003). "Scientists have coasted along on the belief that sexual reproduction is an important factor in survival: that sexual reproduction contributes to diversity and survival of species" (Hortopan, 2004).

However, rotifers speak in contrast to both of these theories.

Scientists at the Marine Biological Laboratory in Woods Hole, Massachusetts have assembled evidence that proves that all rotifers are female. As an all female race, they reproduce asexually by developing unfertilized eggs. After exploring fossil records, scientists have only come to the conclusion that rotifer specimens found in amber from 35 to 40 million years ago, do not include any specimens that are obviously male.

This evidence suggests that rotifers have had a long evolutionary history of asexual reproduction. Research by Meselson and Welch found that by focusing on the same gene in different species that the genes in the asexual rotifers were significantly different, allowing them to come to the conclusion that "there had been no reshuffling of the genome, as occurs during sex, so an individual rotifer's two copies of a gene had each had plenty of time to independently build up quirky mutations" (Milius, 2003).

Rotifers, those scientific contradictions swimming along in waters around the world, may hold the answer to whether or not sexual reproduction is the key to evolutionary success (Hortopan, 2004). Evolutionary biologists have agreed that sex is essential to evolutionary longevity, however they cannot agree why (Milius, 2000). Rotifers seem to be another fly in their ointment.

In the end, researchers have found that nearly identical pairs of genes were found in rotifers that reproduced sexually (nonbdelloid). This result was expected for sexually reproducing diploids. However, these identical genes were not found in bdelloids. "Even the most similar copies found in any bdelloid genome are more divergent than the most divergent pair found in any other rotifer" (Welch & Meselson, 2000).