Jessica Burgess
Biology 1610
Article Title:
The Consequences Of Clone Size For Paternal And Maternal Success In Domestic Apple (Malus x
Domestica)
Article Authors: Matthew B. Routley, Paul Kron, and Brian C. Husband
Abstract:
This study investigates the effect of clone size on reproductive success in apples (Malus x
domestica). Clonal growth (to propagate asexually) in plants can increase pollen and ovule
production per clonal colony (genet). Paternal and Maternal reproductive success may not
increase because within-clone pollination (geitonogamy) can reduce pollen export to adjacent
clones (pollen discounting) and pollen import to the central ramets (pollen limitation). The
relationship between clone size and mating success was investigated using clones of Malus x
domestica at four orchards using blocks of 1-5 rows of trees. Siring rate per clone averaged
70% and did not increase significantly with block size. This was consistent with simulations of
pollen dispersal under pollen discounting. Simulations also indicated that ratio of compatible to
incompatible pollen received by a tree should decline with increased block size from the edges
to the center of blocks. The female function was not significantly reduced among or within
blocks. Results suggest paternal function may be more sensitive to the effects of clonality then
maternal function.
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Introduction:
Clonality is a common mode of reproduction in flowering plants. One consequence of
reproduction by cloning is that genetically identical individuals become spatially (related to,
occupying, or happening in a space) clustered as the clone grows. This structuring can increase
both pollen and ovule production per genet. It can also have negative effects for both male and
female from the transfer of pollen between ramets (direct descendants of a single parent by
asexual reproduction).
In this study using the domestic apple provides these advantages:
1. Varieties are self incompatible, so that fruit is only produced from pollen transfer
between varieties.
2. Each tree of a variety is a ramet of the same genet and genetically identical.
3. Varieties are often structured into single variety clones, or blocks, of different sizes.
4. Previous work has developed diagnostic, genetic markers for many apple varieties for
use in paternity estimation.
This research has three main objectives:
1. In simulations, estimate the theoretical effects of clone size on paternal siring
success in neighboring clones and the potential for pollen limitations within clones.
2. Determine the effect of clone size on outcross siring success in seeds of neighboring
varieties (paternal reproductive success).
3. Determine the effect of clone size on fruit and seed production (maternal
reproductive success).
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Materials and Methods:
Trees tested were from four commercial apple orchards in southern Ontario, Canada. The
apples tested were of the Idared variety. Maternal success was estimated for trees sampled in
each block with these criteria:
1. It contained only Idared trees.
2. It was isolated from other Idared blocks by at least 15 rows.
3. The adjacent row was a variety compatible with Idared with overlapping flowering time.
The fruit was sampled and analyzed for both paternal and maternal success.
Results:
1020 seeds from 34 trees were assayed eletrophoretically (technique of sorting proteins
according to their responses to an electric field) to estimate paternal success in 17 blocks. On
average, Idared blocks sired 70% of the ovules in the adjacent row ranging from 42-74%.
Maternal success was measured in terms of fruit set and seed production. Based on 956 fruits
from 590 branches, fruit set averaged 0.05 and seed production per apple was 5.1. All orchards
varied significantly in fruit set and number of seeds per fruit, but no difference among blocks of
different sizes.
Discussion:
Apples are not naturally clonal. They are not grown in a laboratory, but cultivated in orchards
by means of random pollination. Because apples are not naturally clonal, they may lack some
of the secondary modifications in allocation of resources and floral form that most phalanx
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species have. The advantage of using apples in a study like this, is that the discounting effects
can be separated from the other allocation adjustments that occur with increasing clone size in
clonal organisms. This study also strongly shows the usefulness of decreasing block sizes in
orchards to boost the pollen distribution for more abundant fruit harvest.
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