Tuesday, December 19, 2006

 

Carnal Knowledge: How we evolved into male and female

According to scientists, the very first organisms to dare engage in sex were more like Adam and Steve than Adam and Eve.

That's because sex was invented before heterosexuality - before males or females for that matter.

The first sexual beings to emerge perhaps 2.5 billion years ago were what biologists call isogamous - which is a little like being gay, except everyone is somewhere between male and female.

Many organisms, including some fungi, algae and single-celled pond-swimmers, still practice isogamy*. In doing so they offer clues to the mystery of why and how the sexes ever evolved.

To understand life before the advent of males and females, you need a universal definition of each: Males produce a smaller sex cell (sperm or pollen) than their female counterparts.

Isogamous algae, on the other hand, still have sex but instead of mixing sperm and eggs they mingle sex cells of roughly the same size - generically known as gametes.

What scientists find puzzling is that most of them still use a system of two sexes - in their case plus and minus rather than male and female. Though plus and minus create the equal-sized sex cells, plus mates only with minus and minus with plus.

Such pickiness is an enormous paradox, says Laurence Hurst*, a biologist at the University of Bath. Without sexes, you wouldn't have to limit your choice of a mate to half the population. Anyone else would be fair game.

Continued at "Carnal Knowledge: How we evolved into male and female"

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*From Laurence Hurst's homepage:

"It is not unusual to think in evolutionary terms about the organismic features that are easy to observe: the structure of the finch's beak or the giraffe's neck, the length of time an organism might spend foraging or the number of males a female might prefer to mate with. But can the same sort of thinking be applied to ask questions about the organisation of genetic systems? Current work in molecular genetics is providing information at an ever increasing rate about, for example, how sex is determined, how genes are transmitted, how big they are, where they are on chromosomes etc. Can there also be an evolutionary genomics to allow us to understand these features?

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In a 'Presenters Article' from The Truth Will Out (BBC/The Open University UK), Laurence Hurst addresses the following questions:

What is the trend for age of reproduction and how is this affecting mutation rate?
What effect might later reproduction have on women in the long term?
What will be the effect of modern medicine on the increase of mutation and therefore on selection?
How has globalisation impacted on evolution?
What is the likelihood that humans will speciate?

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*Definition from The Free Dictionary:

isogamy, in biology, a condition in which the sexual cells, or gametes, are of the same form and size and are usually indistinguishable from each other. Many algae and some fungi have isogamous gametes. In most sexual reproduction, as in mammals for example, the ovum is quite larger and of different appearance than the sperm cell. This condition is called anisogamy.

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Just over a month earlier (November 12 2006) Bath University released

"Sperm proteome gives 'tantalising glimpse' towards the origin of sex":

The first ever catalogue of the different types of proteins found in sperm could help reveal the origins of sex and explain some of the mysteries of infertility, say scientists.

Research published in Nature Genetics [1] describes 381 proteins present in sperm of the fruit fly, Drosophila melanogaster. Whilst more proteins may be identified as research progresses, this study marks the first substantial 'whole-cell' characterisation of the protein components of a higher eukaryotic cell (a cell in which all the genetic components are contained within a nucleus).

This so-called 'proteome' contains everything the sperm needs to survive and function correctly, and scientists can use it to investigate the factors that make some sperm more successful than others.

Around half of the genes of the fruit fly sperm proteome have comparable versions in humans and mice, making it a useful model for studying male infertility in mammals.

By comparing the sperm proteome of the fruit fly with other species, scientists will also be able to rewind evolution and work out the core sperm proteome – the most basic constituents a sperm needs for sexual reproduction. This will shed light on how sex itself evolved.

"This is the first catalogue of sperm proteins for any organism, and it offers a tantalising glimpse into how we might begin to answer some of biology's most fundamental questions," said Dr Tim Karr from the University of Bath who led the study.

"Amazingly we know very little about what is in a sperm, which probably explains why we don't really understand sex, let alone how it evolved.

"Before we catalogued the sperm proteome, we only knew a few specific proteins in the Drosophila sperm.

"Being able to compare the structure and content of the proteomes of sperm from different species should help us understand the evolution and origin of sperm.

"We now know of at least 381, which is a greater than 50-fold increase in our knowledge base. Now that we have identified them, we should be able to study the function of all of these."

Proteins carry out an immense range of functions, from forming structural materials to catalysing chemical reactions, so knowing exactly what proteins are in sperm is a great step forward in understanding.

The research involved purifying fruit fly sperm and developing methods to study their protein content. Previous estimates for the protein content of sperm were based on counts of proteins separated into 'spots' on a special gel matrix. However, these only identify the total number of proteins in sperm - rather than identifying the specific identity of each protein constituent

"The sperm proteome provides a basis for studying the critical functional components of sperm required for motility, fertilisation and possibly early embryo development," said Dr Steve Dorus, also from the University of Bath, who collaborated with Dr Karr on the project.

"It should be a valuable tool in the study of infertility as more targeted studies can now be established in model organisms.

"Furthermore, having a comprehensive catalogue of proteins to compare between different species will reveal how natural selection has impacted sperm evolution.

"We can start to look for the 'core' sperm proteome - that is, the most basic required constituents of sperm. This will not only shed light on the evolutionary origins of sperm, but may advance our understanding of the evolution of sex itself."

The research will also help further our understanding of sperm competition – the attributes within a sperm that make one sperm more successful at reaching and fertilising the egg than its peers.

"This question of sperm competition has baffled scientists for years," said Dr Karr.

"If we can work out what makes one sperm more successful than another, we might be able to apply this knowledge to clinical therapies for the treatment of sperm that are not functioning properly."

The findings are particularly timely as a variety of research is beginning to highlight the increasingly important role of sperm.

Scientists are discovering that as well as carrying the DNA that spells out the male's contribution to a new life, sperm carries RNA and proteins which have a direct influence on fertilisation and embryo development.

Professor Geoff Parker, Derby Professor of Zoology at the University of Liverpool, said: "This paper provides a remarkable, pioneering analysis of the molecular basis of sperm form and function by identifying 381 proteins of the Drosophila melanogaster sperm proteome, including mitochondrial, metabolic and cytoskeletal proteins.

"Their work has great relevance to current debate on the evolutionary underpinnings of sperm characteristics, and may have application to mammalian sperm function. The Drosophila sperm proteins show substantial homology with the axoneme accessory structure of mouse sperm."

Professor Manyuan Long, Professor of Genetics and Evolution at the University of Chicago, said: "This is a milestone in the understanding of genomic distribution of male specific proteins. I marvel at their tremendous efforts and great successes."

The research is funded by the Royal Society and the National Science Foundation with additional support from the Biotechnology and Biological Sciences Research Council and the National Institutes of Health.

Source: Bath University PR November 12 2006

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[1] Nature Genetics - 38, 1440 - 1445 (2006)
Published online: 12 November 2006; | doi:10.1038/ng1915

Genomic and functional evolution of the Drosophila melanogaster sperm proteome

Steve Dorus, Scott A Busby, Ursula Gerike, Jeffrey Shabanowitz, Donald F Hunt, and Timothy L Karr

In addition to delivering a haploid genome to the egg, sperm have additional critical functions, including egg activation, origination of the zygote centrosome and delivery of paternal factors. Despite this, existing knowledge of the molecular basis of sperm form and function is limited. We used whole-sperm mass spectrometry to identify 381 proteins of the Drosophila melanogaster sperm proteome (DmSP). This approach identified mitochondrial, metabolic and cytoskeletal proteins, in addition to several new functional categories. We also observed nonrandom genomic clustering of sperm genes and underrepresentation on the X chromosome. Identification of widespread functional constraint on the proteome indicates that sexual selection has had a limited role in the overall evolution of D. melanogaster sperm. The relevance of the DmSP to the study of mammalian sperm function and fertilization mechanisms is demonstrated by the identification of substantial homology between the DmSP and proteins of the mouse axoneme accessory structure.

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Related posts include:"

"Balancing Robustness and Evolvability"

"Is there a homosexuality gene?"

"Evolution First: Parasite Reaches Beyond Host to Play Havoc with Others' Sex Lives"

"Flying sex pest silences the crickets (Hawaii - New Scientist)"

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