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LECTURE OVERVIEW (web, ppt)

Basic statement:

Reproduction insures the continuity of life. In trees, as in other plants, reproduction involves a mixture of strategies. Some reproductive mechanisms foster a diversity of progeny; others insure uniformity. Each strategy has practical and evolutionary implications.

Key ideas:

Reproductive systems of trees can be broadly categorized as sexual or asexual. Sexual reproduction involves the formation of sex cells by meiosis and their union (fertilization) to form the first cell of a new individual. Each step results in new combinations of genes. Thus, sexual reproduction is characterized by a diversity of progeny and is well suited for survival in fluctuating environments. Unlimited variation is disadvantageous, however, and is constrained by several factors (e.g., low chromosome numbers). Asexual reproduction (reproduction without fertilization) results in uniform progeny. The most obvious mechanism of sexual reproduction in trees is propagation via vegetative parts. Asexual reproduction is advantageous in 1) stable environments, 2) conditions in which physical damage is frequent and severe, and 3) harsh environments where sexual reproduction often fails. Plants may have a mixed strategy of both sexual and asexual reproduction.

Important terms:

dioecious	meiosis	gamete	vegetative propagation	stem sprouts
monoecious	mitosis	fertilization	layering	root suckers

LECTURE OUTLINE

INTRODUCTION

-We have discussed reproduction often-- cones, flowers, seeds, root sprouts, root crown sprouts
-Let's organize some of these thoughts today-- put our observations in some sort of framework
-Explore major modes of reproduction in tree

SEXUAL REPRODUCTION

Most widespread form of reproduction-- found in virtually all living organisms

Two step process:

• Meiosis-- formation of sex cells (gametes)


image from http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookmeiosis.html

• cuts number of chromosomes in half (homologous sets of chromosomes from different parents pair at synapse and one set goes to each daughter cell formed in cell division)
• important feature is recombination of genes-- genes (chromosomes) passed on to offspring without regard to how they were inherited from parents

THEREFORE GAMETES HAVE NEW COMBINATIONS OF GENES

• Fertilization-- union of gametes to form 1st cell of new organism (zygote)

• reestablishes "normal" number of chromosomes
• results in new combinations of genes in same cell due to union of 2 different lines of inheritance (mother, father)

 

The above is the "normal" way we think of sexual reproduction and union of gametes-- from two parents with different genetic backgrounds (possibly true for many/most animals-- certainly for humans)

However, plants have nearly a continuum from this approach to the union of gametes coming from the same individual

Advantages of sexual reproduction

• Variety of progeny in the future
• Seeds/fruits are formed- allows for effective long-distance dispersal

Disadvantages of sexual reproduction

• Too much variation is a problem because it prevents close adaptation to any given environment
• Seed germination and seedling stages are precarious due to high mortality rates in stressful environments

ASEXUAL REPRODUCTION: Vegetative propagation-- formation of new individual from vegetative parts of old individual

Types

• Stump or stem sprouts-- adventitious buds (buds formed from other meristematic tissue (such as cambium) on stem or branches (when broken off) e.g Cottonwood, big leaf maple, oaks, willows, American chestnut
• layering-- roots emerge from branches pressed to round. e.g., subalpine fir, redcedar, vine maple
• root crown sprouts-- from top of roots e.g., redwood
• root sprouts, suckers-- upright stems emerge from lateral roots. e.g., trembling aspen, Lombardy poplar, Sassafras

Advantages of Asexual Reproduction

• well-adapted genotypes can be preserved and multiplied
• root sprouts etc take advantage of already existing root system
• no fragile seedling stage \ good for harsh environments
• vigorous regrowth after disturbance

Disadvantages of Asexual Reproduction

• little variation to cope with fluctuating environement (high adaptation, but low adaptibility)
• root sprouts-- disease spreads from old to young individuals of clone \ susceptible for fast spread of epidemics

Practical Uses of Asexual Reproduction

• Mass propagation of unique and desireable genotypes

e.g., Lombardy poplar- 100% asexual reproduction

• Carefully bred genotypes can be propagated e.g. buds of norway spruce are dissected and meristematic tissue grown into seedlings and outplanted
• Sterile hybrids can be mass propagated e.g. P.nigra X P.deltoides is desireable tree-- fast growing, but sterile. One hybrid has been used throughout Europe, but new strain of leaf bacteria is killing this hybrid (disadvantage of monoculture)

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