Biodiversity

Thursday, April 27, 2006

INSECT ATTRACTION

How do insects attract a mate? Different insects use different strategies and what seems attractive to one group goes unnoticed to another. But they all serve the same purpose: to find a mate and complete their life cycle. Here are a few examples of the diversity in attraction methods that have evolved from colour, to smell to sound.

Butterflies and moths are the adult stage of a group of insects called the Lepidoptera. The sole purpose of this stage is to find a suitable mate, sexually reproduce and secure the maximum amount of offspring. There is no simple distinction between moths and butterflies, but generally moths are mostly nocturnal and their larvae construct intricate cocoons to protect the pupae. The wondrous colours of most butterflies and some diurnal moths are produced by small scales that cover the wings and body. “These scales have pigments that produce the white, red, yellow and orange colours with the brighter metallic and iridescent colours being produced by fine grooves and ridges on the scales that diffract light.” (1) In some species the males have modified scales in the form of long scent hairs that release chemicals to attract females during courtship.(1) This is a species specific communication, where females respond only to the chemicals release by males of their own species. (1)

Wing patterns and colours in butterflies play a role in attracting a mate. In some species the specific wing patterns allow a mate to recognize a partner of the same species. (2) In others it seems that the brightness of the colours influences the selection of a mate. (3) Some species have ultra-violet reflecting colours which indicate their sex allowing females to recognize a male and males to recognize each other. (4) Polarized light for mate recognition is used by some species of butterfly that inhabit deep forests where lighting can be tricky. (5) But is not only colour that is used to attract the opposite sex. Many butterflies perform elaborate courtship displays to attract females.

Moths use pheromones to attract each other. The sex pheromones are produced and released by the female moths to advertise their readiness to mate. These sex pheromones can be picked up by male moths over very long distances. They have thus adapted a “smell strategy” compared to the butterflies, who have perfected their “colour strategy”.

Insects in the order Orthoptera use sound as their attraction strategy. Grasshoppers and locusts produce sounds with special sound producing organs on their hind legs.(6) They do this by rubbing these special organs on their hind legs against the strong veins of their wings. Crickets and katydids have specialised sound organs on their wings.(6) With these organs the katydids produce a sound that sounds like “Katy did, Katy didn’t”, hence its name. (7)
Cicadas are of the order Hemiptera. The male cicadas also have specialized sound organs and are the master singers of all insects. These organs produce the sound through vibrating the membranes with the help of strong muscles. They have two hollow resonators in their abdomen that amplifies the sound, and some species have the capability of producing sounds exceeding an incredible 106 decibels.(8)

References:


1. Picker M, Griffiths C, Weaving A. 2002. Field Guide to Insects of South Africa. Cape Town: Struik Publishers. Page 314

2. Fordyce JA, Nice CC, Forister ML, Shapiro AM. 2002. The significance of wing pattern diversity in the Lycaenidae: mate discrimination by two recently diverged species. J. Evol. Biol. 15: 871–879.

3. Knüttel H, Fiedler K. 2001. Host-plant-derived variation in ultraviolet wing patterns influences mate selection by male butterflies. J. Exp. Biol. 204: 2447–2459.

4. Silberglied R, Taylor O. 1978. Ultraviolet reflection and its behavioral role in the courtship of the sulfur butterflies Colias eurytheme and C. philodice (Lepidoptera, Pieridae). Behavioral Ecology and Sociobiology 3: 203-243

5. Sweeny A, Jiggins C, Johnsen S. 2003. Nature 423: 31

6. Picker M, Griffiths C, Weaving A. 2002. Field Guide to Insects of South Africa. Cape Town: Struik Publishers. Page 74

7. Wikipedia contributors. Katydid [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 27, 02:10 UTC [cited 2006 Apr 27]. Available from: http://en.wikipedia.org/w/index.php?title=Katydid&oldid=50357634.

8. Wikipedia contributors. Cicada [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 27, 12:00 UTC [cited 2006 Apr 27]. Available from: http://en.wikipedia.org/w/index.php?title=Cicada&oldid=50409567.

Karen Marais

E-mail 2657211@uwc.ac.za

Web http://brit-journal.com/karen2006bcbnisl/

THE INTERACTION BETWEEN THE FIRST PLANTS AND ANIMALS

Terrestrial plants first evolved without flowers. The first land plants evolved around 400 million years ago and were probably mosses. They used a mechanism of a sexual and an asexual cycle to reproduce. Then at around 350 million years ago the cycads came along and with them a different reproductive mechanism developed. They evolved into male and female plants, both producing a huge central cone, which evolved from the previous spores. The male plants produced huge amounts of pollen which is mostly wind dispersed. But little insects also started feasting on the nutritious pollen and along with that a strategy developed to attract these creatures that were now helping the process of pollen dispersal. One species of cycad even today practises a strategy where the temperature in the cone is raised by about 2 degrees when the pollen is ready for distribution. This attracts weevils, which come to feast on the pollen, get themselves covered in the process and then deliver the pollen to another cycad in search of another meal. This way of pollen dispersal is a lot more economical than the wind and both the cycads and the weevils are winners (1).

It is only about 100 million years ago when the first flowering plants evolved. This was an advertisement strategy and still is today. Pollen is nutritious and insects long learned to utilise this food source. Now plants developed more strategies to attract the pollinators and the flowering plants were born. Water-lilies and magnolias are descendants of some of the oldest plant families that first produced true flowers. They still reward the little beetles that visit them with little more than the pollen, but this has served to be successful enough for these plants to be around still today.

Some plants developed an exclusive relationship with just one species of insect. One such example present today is of the pink Orphiump frutescence that is only pollinated by carpenter bees. The flower holds its pollen inside its hollow anthers and the only way the pollen can escape is through a tiny hole at the top of the anther. The carpenter bee has perfected its technique of extracting the pollen by alighting on an anther and then beating its wings at a certain frequency, just right to make the pollen spout out of the hole at the top. As only the carpenter bees can buzz at this frequency, they alone can extract the pollen. They however do not know if a flower has been visited before and so they move from flower to flower, shaking the anthers and in the process pollinating the flowers with the pollen that has collected on their furry bodies (2).

Another such example of a specialized partnership is seen in the twinspurs (Diascia) in South Africa. There reward is oil which is secreted at the far end of the spurs. Several species of solitary bees have developed brushes on there forelegs to collect the oil. For each species of twinspur there is a corresponding species of oil-collecting bee with forelegs that exactly match the length of the spurs (1).

Some flowers started producing nectar as a reward to attract insects and some produced scented chemical to attract their pollen couriers and yet another advertisement is colour. Many even developed “landing strip marks” or nectar guides that help the insects guide their tongues into the flowers to collect their reward and often also parcels of pollen which they then carry to the next flower.

Some plants even mimic others to try and trick a pollinator to visit. One such example is the cluster disa (Disa ferruginea) on Table Mountain. It has a bright red colour and closely resembles the red Tritoniopsis triticea (alongside it also grows), which produces a rich nectar reward. The disa however produces none, but manages to trick the mountain pride butterfly who repeatedly visits the empty flowers and thereby pollinating it (2).

There are many more examples of how plants and insects have evolved together, adapting together to serve each other. Some are extremely bizarre, but all serving a purpose of fulfilling a life-cycle, all part of a functional ecosystem.

References:

1. Attenborough D. 1995. The Private Life of Plants. Pages 95-106 in Flowering. London: BBC Books.
2. Pauw A, Johnson S. 1999. Table Mountain. A Natural History. Pages 55-67 in Delicate Partnerships. Cape Town: Fernwood Press.

Karen Marais

E-mail 2657211@uwc.ac.za

Web http://brit-journal.com/karen2006bcbnisl/

THE STRUCTURE AND SIGNIFICANCE OF DNA TO LIFE AS WE KNOW IT

DNA is a chemical known as deoxyribonucleic acid. It is the key to life, the master molecule that contains the recipes for producing proteins that regulate all functions in an organism, from the activities and repair of individual cells to the development and function of organs systems, even to reproduction of the organism itself.

In eukaryotes (1), DNA is found in the cell nucleus where it is spiralled up in its typical double helix form. Most eukaryotes also have mitochondria, which contains its own DNA. In plants the DNA is also stored in chloroplasts. In prokaryotes (2) (mostly bacteria), cells without a nucleus, the DNA is stored in a single circular chromosome in the region of the nucleoid structure, but smaller circular pieces of DNA can also be spread throughout the cytoplasm.

DNA contains the genetic code or recipe to produce proteins (made up of amino acids), which regulate the development and growth of any living organism. DNA resembles a twisted ladder (double helix), its sides made up of linked phosphate and deoxyribose (sugar) molecules. The rungs of the “ladder” are provided by nucleotides that link the two sugar chains. There are 4 nucleotides namely adenine (A), cytosine (C), guanine (G) and thymine (T) and they combine in fixed combinations to form base pairs. “A” can only pair up with “T” and “C” can only pair up with “G”. Three base pairs together form a codon that codes for a specific amino acid, and amino acids are the basic building blocks of proteins. Each DNA strand has directionality and therefore the sequence of the nucleotides matters. “T+A” is therefore not the same as “A+T”. In this way DNA contains the master code that regulates the production of every single protein produced by an organism.

A DNA strands contains genes that code for a specific protein, regulatory areas that control the function of these genes, and areas where the function is not yet known. Genes exist as pairs or alleles, such that there are two doses for each genetic determinant, one from the mother and one from the father. Alleles code for specific hereditary traits and have many viable combinations, for example the allele that determines the colour of a flower petal has different possibilities, giving rise to different coloured flowers in the same species. In this way DNA provides the basis for transferring specific genetic traits to the next generation through inheritance.

Living cells usually divide on a regular basis, to achieve organism growth and to replace damaged cells. This type of cell division is called mitosis, but before mitosis can occur, the DNA must be replicated. Replication occurs when the DNA molecule unzips itself along its length and separates. The two single strands then act as templates for the formation of two new complementary strains. As a result, two identical copies of the original DNA molecule are produced. Once the DNA (chromosomes) has been replicated, each of the individual chromosomes line up on the equatorial plain. Their doubled structure is clearly visible as two identical chromatids. Cell division occurs when microtubules pull each of these chromatids to an opposite pole of the cell, before division of the cytoplasm takes place. In this way two identical daughter cells are formed that contain a full set of chromosomes.

Sperm cells and egg cells are produced by a different mechanism called meiosis, during which chromosomes align themselves in pairs (each consisting of two chromatids) on the equatorial plain. At this time, random genetic exchange occurs between the chromatids of the different chromosome pairs. After this process of genetic exchange, members of each chromosome pair are pulled to opposite poles of the cell before the cytoplasm divides. After the first meiotic division, each daughter cell contains only half the complete (diploid) number of chromosomes. The second meiotic division is similar to mitosis, but the end result (due to the genetic exchange that occurred) is that 4 daughter cells are formed with only half the chromosome number (haploid), but with unique genetic rearrangements.

During the process of DNA replication, when a DNA strand unzips itself to be duplicated and form two new strands, mistakes (mutations) may occur (mutations may also occur at other times). These mutations generate genetic variability, giving rise to new traits and a multitude of new possibilities. Therefore the genetic variability in organisms, which is essential for evolution to occur, results from genetic exchange during sexual reproduction and from random DNA mutations that may occur at any time.

These two processes, genetic exchange that occurs mainly during sexual reproduction (genetic exchange may also occur without sexual reproduction as happens between bacteria) and mutations, create the genetic diversity that underlies phenotypic variability. The creation of variability is an essential ingredient of natural selection, when a strong selective force constantly identifies the fittest individuals.

DNA is thus truly the recipe book for all living oranisms.

References:

1. Wikipedia contributors. Eukaryote [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 25, 16:15 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Eukaryote&oldid=50104670.
2. Wikipedia contributors. Prokaryote [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 17, 10:15 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=Prokaryote&oldid=48824695.

I used the following site as my general source:

Wikipedia contributors. DNA [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 26, 11:37 UTC [cited 2006 Apr 26]. Available from: http://en.wikipedia.org/w/index.php?title=DNA&oldid=50241434

Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17
Bellville

E-mail 2657211@uwc.ac.za
Web http://brit-journal.com/karen2006bcbnisl/